Hmmmm. The thing with upgrading is you have two choices. You can upgrade regularly, to stay on top of the latest hardware or you can sit with your hardware for two-three years and then just have one huge ass upgrade at the end of.
Personally, i prefer the upgrade-regularly route. For a number of reasons, really:
1) If that money for a new rig was to sit in my account for two-three years, it would get spent on other stuff first.
2) I like playing all the latest games at max detail, and more importantly, with decent FPS.
3) If you're smart about it, you can get brand new, next generation hardware with a very small pay in. Just need to time the sale of the old hardware properly.
4) It's like having christmas. Twice or three times a year
5) If you upgrade regularly, you tend to stay on top of hardware releases and know what is the best bang-for-buck at the time. If you're out of the market for two years, it takes quite a long time to get caught up on the latest hardware, which necessitates a lot of sitting down and reading. Generally, i don't have free time in such large quantities.
Anyway, my current plan is to upgrade my 2 X 4890s to 2 X 5870s. Very interested to see if my current processor bottlenecks the cards badly. If so, i'll have to upgrade the cpu/mobo/ram too. Oh well.