This is a very confusing question in a lot of ways. For example, a video card upgrade can be very costly, or quite cheap, and either way it might improve performance a lot, or not. Even a CPU/MB upgrade might not be all that expensive, especially if the system is already loaded with periphals. Especially for the longer intervals, a lot of people's upgrade intervals will be inconsistant. And now with the rate of improvement slowing down so much, I think it'll either discourage people(at least those with sense) from upgrading/replacing so frequently, or make people switch from replament-upgrades to stacking upgrades. And this will be far more true for those who are suspicious that paladium will end up controlling what programs they can and can't run on future systems.
I had four systems, 33, 166, 333 and 1663(2000+)mhz. With the first three, I only made minor upgrades (doubled ram on every one, multimedia kit( remember those?), 256k VGA to 1M, and a voodoo1 card). This one, however, I think I'll upgrade a lot. I've considered upgrading pretty much every way possible.
With the heat wall stopping CPUs, what I'm most eagerly awaiting is for desktop hard drives to start moving to 2.5". Maybe then I'll switch to a smaller form factor.
Edit: think I'll add a complementary poll