In my opinion, every game should be re-bought to play on the PC. May I ask the system you are buying?

Heres a couple of games:

GTA 4, Crysis and Crysis Warhead, Fallout 3, Left 4 Dead, Unreal Tournament 3, TES: Oblivion, Cod 4 or 5 ( I personally enjoy Cod 4 better), The Orange Box, which you said you're buying, The Sims 3 (Next month) If you are a sports fan, then, NBA 2k9, Pro Evolution Soccer 2009. If driving games are more your thing, Race Driver: Grid, DiRT, Burnout Paradise the Ultimate Box. For RTS Games, Warhammer 40k: Dawn of War 2, World in Conflict (Brilliant game), Company of Heroes: Tales of Valor, Empire Total War. Some classics include KOTOR, Quake 3, Unreal Tournament 2k4, AoE 1,2,3, Starcraft and Counterstrike (Still being played today).

Hope I could help :)