I'm currently playing the Dungeon Siege expansion "Legends of Aranna" (in chapter 3 now I think), and only recently I learned about these "sets" that grant you perks the more you have items in a set:

http://ds.heavengames.com/library/sets.html

Does it make much sense to try to hunt them down? The thing is, I have already sold e.g. the armor and bow from e.g. the "Arhok's Lot", and I don't feel like backtracking to meet every shopkeeper, trying to buy it back (I don't recall to whom I sold them).

Will I get much better armor and weapons anyway so these sets become obsolete anyway? Yeah I guess it is nice you can get e.g. 10 + 54 armor from completing the Arhok's Lot set, but I presume I will later find other armor that grant much more armor anyway?

Ok, that's a relief. I was a bit angry I had sold some of those pieces without fully understanding the concept of "sets", but reading that web page didn't make at least that first set look that great. I even started running backwards towards the start of the game, trying to find every shopkeeper to check to whom I had sold some set pieces, but I found that too tiresome.

Maybe indeed some of the more magic-orientated sets are better, giving lots of extra mana etc. I'll need tp keep my eyes on the rest. I started the expansion pack from a scratch. In the base game I decided to go with a 8 member party, even if it meant less XP per member. Four melee fighter tanks, one archer, one combat magic and two nature magicians (because some fights were so intense that having two healing magicians helped a lot, and other times nature magicians can fight with their spells too).

I think this time I'd probably have three melee fighters and two archers, because at least in the base game there were a couple of encounters which were better with archers. For instance those "beholder"-type of floating heads with one eye, and one boss dragon. They seemed to cause so much more damage close, that it was a much better idea to try to stay away from them and shoot at them afar. So occasionally I even told my melee fighters to stand back, and sent only my one archer to shoot at the enemy, and one or two nature magicians constantly healing him.

Also there were some dungeons where enemies kept shooting from other rooms through grates, trying to fight them with melee fighters was frustrating as you had to run around to get to them. With archers and magicians, you could fight them through the grates.

I even thought about having no melee fighters at all and instead having five archers (and one combat magic and 2x nature magic), but I guess some other fights might become too hard without melee tanks...

I completed them for lore reasons. It felt kinda good vanquishing foes in your parents gear.

Edit: Always have a secondary ranged weapon set for every character. Makes playing a whole lot easier.

Yeah, but in those cases where I needed to use ranged attacks (that one "scorch" dragon, and those beholders, also the bigger ones), it actually worked better if I used only one or maybe two archers.

The reason was that my two "healers" could quite easily keep healing one or two characters, but if there were e.g. six characters getting damage constantly, their healing abilities weren't enough.

Archers aren't great after a while.
Sadly dexterity only improves aim, but not damage.
But warrior-archers work quite well, if you keep strenght ahead.

About magic, I prefer nature to combat.
It has much more convenient spells for healing, buff, summon & attack overall.
The only problem is that you have to constantly switch attack\healing to get decent XP (unless you use that mod to auto-casts healing at X% health).

Does strength affect only melee damage, or also archer damage? I think archers still get extra strength too, but not as fast as melee fighters.

I decided to go with fully specialized characters though, especially since I intend to have a full 8 member party (less XP per member). I think I read some FAQ which seemed to indicate that the game also punishes you if you try to dual-train your characters, like that your weaker/secondary skill will advance much slower anyway. Not sure how though, or if I misunderstood something. I tend to agree, many instructions etc. indicated that "combat magic is better at magic attacks", but at least I didn't see that much advantage there. It just seemed that while combat magicians have fire-based attack spells and such, the nature magicians have ice and electric attacks. Not sure if there is more variation in very high spells, but my combat magician didn't get high enough for the most bad-ass spells I guess. I think near the end of the base game my combat magician got just enough levels to use some "fire rain" spell, but even that was pretty mediocre.

And like you said, nature magicians seem better at healing (even if combat magic has some healing spells too). Having two nature magicians which I could use as healers in intense fights (and between), I hardly ever had to use any healing potions, I usually just sold the ones I found.

Anyway, I am keeping a combat magician around just to spice things up.

EDIT: That reminded me that when I said earlier I used a single archer to kill some of those enemies/dragons which caused lots of melee damage, I guess another option would have been to use my three spellcasters to constantly send summoned monsters at them. I did that for awhile too with that dragon in the base game (who has an insane amount of HP so you have lots of time to experiment attacks with it, as it doesn't even move), but sometimes the problem was that the summoned creatures wouldn't attack, but just come back to the spellcaster. I guess you needed to summon them close enough to the enemy.

Damage is everything in this game because there are so many enemies. Better focus on getting rid of the crowd than try to outlast them.

Yeah generally so I guess, but with that dragon and the beholders, I had a problem with my melee fighters because it seemed those enemies cause exponentially more damage the closer you get to them. My four melee fighters could deal quite a lot of damage and had good armor, but they would last maybe 10 seconds against those enemies, even if I kept hitting the "drink health potion" button (which unfortunately also causes a pause in their attacks).

With ranged attacks (archers, maybe summoning monsters etc.) I could last well as they received much less damage due to the distance, and it was also easy to flee the fight in case you seemed to have a problem. With melee fighters, trying to flee when you were next to the strong enemies would just cause them to die.

Then again, the problem with using archers was that defeating the dragon itself takes an insane amount of time, as it has so much HP and even a good archer deals only some damage. So I used quite a lot of time with that dragon both hitting it with my archer and summoning monsters against it (the latter actually worked better IIRC, some of the summoned monsters dealt quite good damage to it; but unfortunately sometimes the summoned monsters decided to just come back to the spellcasters, instead of attacking the enemy), and when it got a bit lower in health, I finally sent also my melee fighters to finish it off quickly.

Note: I play the game in the hardest difficulty level, if that matters.

Note2: One problem also might have been that I never really used "buff magic" to e.g. make my melee fighters stronger. One reason was that I was just too lazy to use most of the single cast spells (I preferred using spells which the magicians would use automatically, and I didn't have to cast them manually). The other was that, well, to me the buff spells that I had didn't seem that great anyway. "Gives +1 strength to all party members for 10 seconds", umm ok, I think I try to survive without.

I think only melee damage.
Each skill increases more its primary attibute, less its secondary one, and a tiny bit the rest.
I probably saved the % somewhere, if you really want it. With multi-classes you won't reach attibutes' max stats, but, if done properly, you will obtain quite versatile characters and still be able to wield\use the highest level useful weapon\spell.
Each member of my party was 2 classes miminum and my main character 4 lol. Yeah, the first worthy attack CM spell after Fireball (lvl20) is the slow and expensive Flame Blades (lvl52!).
In DS you often get "stuck" on the same spells for a lot of time. I remember some stationary dragons-serpents that came out of the water and spat fire at you.
I used exactly the summons trick to kill them, because I couldn't survive attacking them directly in any way.

The secret is a mage using Healing Wind :)
I used it for a long time (till I could use Merik's Windfall + Major Healing).
It's cheap, low level and very effective, compared to the rest.
You just have to remember that it takes a pair of seconds to trigger.

To defeat the dragon (or similar bosses), I left 2 mages in the rear dedicated only to healing, while the others attacked it.
I had to retreat & heal (& save) various times, though. I made your same error, hoping to obtain better loot.
Then I discovered at the end of the first campaign that the difficulty doesn't increase the drop quality at all. :(
It only gives you less XP (=lower hero level!) and makes enemies stronger.. basically masochism XD I strongly suggest the autocast mod, it makes buff spells finally manageable (how could the devs expect players to constantly cast them every N seconds?)