Why I want to wait:
1) I haven't gotten a game yet that isn't at least playable on my (not quite antiquated) system, so I can hold off on the upgrade for a little while longer. Besides, I'll be able to get more CPU and memory horsepower for the buck that way!
2) I own Nvidia stock. Like I'm going to contribute money to their competition!
Historically for me, it's always been a piece of software that pushes the upgrade. The Star Wars Screen Entertainment package pushed me from a 386 to a 486. Quake (and a free processor) pushed me from there to a Pentium Pro. Shogun: Total War pushed me to my current system. And I imagine it'll be Doom 3 that pushes the next upgrade (unless the Raven Shield or Splinter Cell requirements exceed the wooly mammoth that I call a computer).