Oblivion: Game of the Year - I'll finally be able to play the game the way it was meant to be seen!
Fallout 3 - never played, but highly recommended and same company that put out Oblivion.
Gran Turismo 5 Prologue - I wanted a driving game and everyone said this one was to get, in spite of the fact the full Gran Turismo 5 is out in a couple months.
Tekken 6 I love fighting games.
Soul Calibur 4 - more figthiing - I had tekken and soul calbur on Xbox (as well as dead or alive)
Oblivion might just be my all-time favorite video game. Fallout 3 is up there too. I also loved Morrowind (Elder Scrolls III) and almost didn't buy it (I played it after Oblivion, Elders Scrolls IV) but my buddy who got me into Oblivion told me it was worthwhile, and it was. A few sessions into Morrowind I stopped noticing how much more primitive it looked.
If Bethesda released an Elder Scrolls V and charged $500 for it, I'd be first in line.
I also rate Gran Turismo 3 and 4 pretty highly. Buying GT 5 Prologue strikes me as a total waste of money, considering it's something like 2% of the content of the full version of GT 5, for about 50% of the price (unless of course you found it really cheap used somewhere). I mean, I bought Prologue myself when it came out, but that was because I was dying for a GT racing fix and the complete game was more than a year away. But I sold Prologue a while back because I figured nobody would want to buy it once the full GT 5 had a firm release date.
Not interested in fighting or sports games (except racing) personally, but I know lots of gamers love these.