From a realistic standpoint, Zedd's absolutely right. I was being a little flippant, and then forgot to be reasonable, lol.
If you want to upgrade the CPU, I'd try ebaying a Dual 1 GHz or so.... new HDDs are never a bad investment since you can plunk them anywhere. RAM you can't, but it's cheap, so go for it if you need it.
Anything past that, I'd go for a G5. You can get them cheap now.... a little over $1000 for a dual 2 GHz, I think.