I have always wanted to play Wyvern, Darkness and Tyrants, but never able to get them. If I buy them here at GOG, will they install and play on my original NWN 1, that was from a DVD and not purchased on GOG, and is not the EE version?

Sadly, no. You'd have to run them with the EE. And I also think you'd have to buy the EE in order to buy the DLC, GOG usually doesn't allow you to buy DLC to games you do not own here.

Btw, the three official former Premium modules are Wyvern Crown, Pirates of the Sword Coast and Infinite Dungeons. Darkness and Tyrants were never available for purchase before the EE. As Premium modules they were cancelled, and the authors put these campaigns up for free on the NWN Vault. You can still legally download them for free and play them with your old version (provided your DVD includes the expansions, SoU and HotU). It's just that the commercial versions for the EE were expanded and enhanced, and voice-overs were added.

You can find the old, free versions here:
Darkness Over Daggerford
Tyrants of the Moonsea

Thank you Leroux - I noticed that EE version was on sale, so for $4.99 I just went ahead and bought it and the other 3 for a few dollars. I'm happy now, and I can play NWN in a window!

How are the old free versions of those 2 DLC's (Darkness and Tyrants) when compared to the redone/remastered new EE versions of those?

http://www.ossianstudios.com/downloads/DoD_Enhanced_v2.0_Change_Log.txt
Mostly Voice acting and tweaks.

I expect similar for Tyrants, but Darness seems better received.

Hm, I've played all four, but the free ones about a decade ago, so I don't remember every detail. Like Peter Scott said, the DLCs have some voice-acting while IIRC the original free modules have none. Apart from that, I think Darkness Over Daggerford EE has some additional dialogues and quests, but I couldn't say which ones. Tyrants of the Moonsea also features new tilesets, two or three new creature models, a BG2-style overland map (the free Darkness Over Daggerford already had one, too, but I think the free Tyrants did not). It's possible that the DLC is also longer and more complete than the free original, probably includes a few new dialogues and quests, too. Possibly new music.

I think at least for those two DLCs, the term "enhanced" is justified. But if you don't want to buy the EE (which arguably isn't all that "enhanced") exclusively for playing them, the originals are worth playing, too.