You can pick any save to import but if you import a Lv60 then you get more bonuses. As for the choices that count I would say these are either important for ME2 or ME3:

Allowing the Rachni Queen to leave.
Saving the Council
Wrex survived Virmire
Giving the Geth tech to Tali

I would say those are the most important. Pretty much every choice you make is recorded but it only seems to affect e-mails you get or NPC's you encounter. Also the choice of Anderson or Udina is not recorded and you set that at the beginning of ME2 via a conversation. Bring down the Sky is free on the PC not on 360. It's worth it though, unlike Pinnacle Station which was a total waste of money.

*** Mass Effect SPOILER ***

I guess I missed that one, I assume she'd use it as a gift for her Pilgrimage. Do the Rachni figure prominently in ME2 or are you making a guess at ME3?

Detailed info on what gets transfered can be found over at the ME Wiki

Without wanting to spoil anything you meet an Asari who gives you a message of thanks from the Queen and lets you know they will be ready to fight the Reapers with you. I IS FORSHADOWING.

To avoid spoiler territory... will see in ME3 but until now most choice have very limited actual impact in ME2.

Most of them results in different background chatter,radio news announcement, or e-mail you receive (peoples thanking you for saving them things like that).

Even the "bigger" decisions don't have that much impact in the end, yes some scene are different depending of who live or who died, sometime instead of NPC A giving you the mission it will be NPC B who happen to be at the exact same spot, it gives you the illusion that what you did had an impact, which is nice, but game play wise don't expect any missions to actually change or anything.

I hope that as episode three is the "last" one you decisions will have a bigger lasting impact.

Well they can't hammer the message home any harder it even mentions your choices will affect ME3 on some of the loading screens.

I think most people are convinced it will affect the kind of army you will be able to field against the Reapers and the overall difficulty. Having the Rachni and united Krogan's under Wrex and Grunt's leadership will no doubt we a massive help as will making peace between the Quarians and the Geth. My last Paragon Shep is certainly ready for ME3 now the bonus of having the new Shadow Broker on his side. Well everyone is behind him except for that stupid finger waving Turian on the council of course.

But doing so could easily break the gameplay... I guess said armies will just make some apparence during some big space battle ... big battle for which the outcome will be decided solely by some suicide mission that only you can undertake.

Personally I hope that depending of you choice it will really impact the story line like changing the objective of some mission or even making some missions available and others unavailable.

But given the amount of choices you can make, the number of character that can potentially die or live I am a little worried.

I'm practical minded. I'd go for Lost Horizon, even if i've never played it before and don't know if it's any good, but you've got 30% of it done already. By the time you get back to any of the other two games, you can easily restart them from scratch if you want to without having to replay much.

The hard part for me about restarting a game I've left idle for too long is getting used to the controls again and remembering what i had done of the story so far. The farther in you went, the harder the game got and when you return to it you might be at a loss and losing all the time. Plus, you only have 70% to go right now, it should be the shortest to finish of the bunch (unless the campaign is long).

Not really it will just be a lot harder if you were Renegade. Remember if you didn't save the Council it's seen as a good thing from a human POV. If you played Renegade then I can imagine you have Clone Krogans, Cerberus experiments and a really strong Geth to aid you.

Yeah, they could just give you different ally choices based on your imports. A bonus ally would be kind of fun story-wise and not gamebreaking.