It'd cost you about the same probably....
But....
if you wait 3 years to buy a new computer...
Lets say you start at point A
500MHZ PIII
8GIG HD
128 MB Ram
etc
go to Point B
1.2 Ghz Pentium 4
20 GIG HD
256 MB RAM
And end at Point C
AMD Athlon 2500+XP
80GIG HD
512 MB RAM
Person A upgrades every once in a while...having upgraded periodically, and Ends up with his final computer at the same time Person B does. Person B waits 3 years to buy a new computer.
While Person B waits 3 years for his new upgraded HD, and more RAM...Person A has been using it for a year or so. Sure, it may be less hassle to buy a new comp every 3 years, but you'd be shut out from using various things. (Not sure what the XP requirements are, but I don't think Computer A would run it too well.)
So, comp A could be (possibly) shut out from using XP. Person A upgrades his Mobo/CPU for 200 bucks...and is able to use XP.
Then, along comes some game he wants to play....buy a new video card...100 bucks (just a random figure), and is able to play...while person B still can't even get on XP.
Anyway...I've had 3 computers since 96. I upgraded minor things here and there on everyone. Although, if I had the money, I'd definately have upgraded. Being stuck on a P3 in 2002 sucked when I could barely play Max Payne.