Video card! It's the easiest... cheapest... and a lot of multiplatform games that look good are also going to be on PC.
Borderlands/Prototype/Left4Dead
and Spore... and those games you mentioned.
Edit: Ah too late. Still probably the best choice. PC seems like it may end up the winner in the world of multiplats... since if it's going to be a 360/PS3 multiplat why not put it on PC too?








