I think you all need to admit that sometimes it's not worth bothering. Replacing parts can get expensive unless you have a source of really cheap stuff that still works. And you're still left with a very obsolete computer that any new one, even the cheapest, will run rings around. There's a strong tendency to throw good money after bad--"if I just buy a little more RAM for $50, it'll run better; if I just buy a new hard drive, I won't have to fight for space; that noisy fan is getting too annoying--I'll buy a new one." But it's still a 500 MHz antique.
I have a history of that kind of thinking and doing, but I think I've finally learned my lesson. Now when a computer gets too slow for me, I give it to some organization or person who wants it and spare myself the suffering.
That said, my oldest computer, a 1.3 GHz desktop, runs admirably with VectorLinux despite being years behind the curve and definitely doesn't need replacement.