Far Cry: I have this game and its really cool. It has a lower price now, and the graphics are pretty amazing. Lots of tropical environments.
Republic Commando: I have this game as well. Its pretty cool, but the single player campagn is very short and the multiplayer isn't that great.
Half-Life 2: Do not have this game, but I should. This has the best graphics out of all of them and CounterStrike is VERY fun to play online. I would definitely suggest this one.
UT2004: Never really got into it...
Halo (PC): Well, you can't really go wrong with this. The single player will last you for a while, but the multiplayer is where the fun is. That never gets old.
Max Payne 2: If you are willing to get a third person shooter I would suggest this game. It has very good graphics, plus a gritty crime plot. The bullet-time is flawless also.