If you buy the best you won't need to upgrade!!
That's why I won't be buying a new videocard for a long time (probably 2-3 years!). My videocard was $430, but it's also the best in the world right now. You can't upgrade from the best!
The only thing I'd upgrade on my PC is the motherboard, processor, and RAM. I don't have the money for it right now (and I won't for a long time) but if I did then that's where it'd go. My Barton 2500+ is still kickin strong and I do have 768 MB of RAM but I'd still like a shiny new Athlon 64.
I suppose my point is that if you upgrading to the best of what you *need* then you're fine. Today's games rely on the videocard and the amount of RAM you have way more than your processor so I don't have a whore-myself-out obsession with upgrading my processor.
-Lasereth