You better have tons of time if you want something like Guild Wars. I'd recommend Diablo 3 if you like dungeon crawlers, but it's back up to full price after being $10 at the Microsoft Store a few days ago. Planetside 2 just released on Steam and is free and very good. Amnesia is another good one if you like psychological horror.

If your card has PhysX support, Borderlands 2. The effects are awesome.