Check out macsales.com. It appears that they have processors and memory for that model of the G3 you're looking at.
You might also consider a G4 cube, which was the next generation, if I'm not mistaken. The G4's are pretty slick and were very quiet as they did not have fans to cool the internal components. You can also upgrade those, but might run into thermal issues.
But if you're talking about upgrading, it maybe be better to consider purchasing a mac mini (about to be discontinued, according to mactactic.com - so that's kind of iffy), imac or macbook, depending on your needs. To me, upgrading really only makes sense if you already have the machine and want to extend it's useful life further.
If your heart's set on that G3, remember, it like everything else on ebay - caveat emptor.