That obviously depends on a lot of things. What kind of upgrades would you plan on adding? How much does a desirable (in your opinion) new computer cost?
I know it's quite dependant, but I was thinking: "I would already have most of the stuff, I'd just need to buy a few new things like RAM, video card, and (according to that lame commercial) a new proccessor. Assuming that all fits on your motherboard, that's less than an equivilant new computer, yeah?"
So, that's just what I was basing my oppinion on.