I'd recommend these from your list:
Icewind Dale 2 Complete
Icewind Dale Complete
Neverwinter Nights: Diamond Edition
Arcanum: Of Steamworks and Magick Obscura
Baldur`s Gate: The Original Saga
Baldur`s Gate 2 Complete
Planescape: Torment
Alpha Centauri
Dungeon Keeper 2
Master of Orion
Temple of Elemental Evil
Not on your list I would recommend
Underworld 1 and 2 AND/OR Arx Fatalis (RPG)
Dark Reign and expansion (RTS) (instead of TA)
Darklands (RPG)
That would have you spending about $42
ToXiN: Thanks for all the replies! I am a big Bioware fan, Knights of the Old Republic is what got me into wRPG's, and I have loved pretty much every game Ive played by them (well, DA2 was a bit disappointing but still a good game). I also have Sacrifice, but never really go into it. And I don't really want any newer 3D RPGs in the vein of Gothic or Elder Scrolls, I have too much of those anyway.
I now think I will swap out Lionheart, for either Master of Orion or Temple of Elemental Evil.
Also, could you guys tell me more about Betrayal at Krondor? I have never heard of it before, but it looks intriguing. Specifically, what is the gameplay like?
You Tube is your friend if you want to find out about any particular game on GOG or being recommended here! :) For example here's a great video that explains Betrayal at Krondor
http://www.youtube.com/watch?v=W0HLoKTJOhA: