If you are into any of those things, and don't want to mess around with installing motherboards, power supplies and RAM, your best bet is probably to buy a "barebones" box which has mobo, RAM, power already installed in a new case. Swapping over HDD, CD, sound and video boards from your old machine should be a slam dunk for you (although if you are getting into gaming you'll want a new video board, at least a GeForce 3).
I can recommend a couple of barebones box vendors that have done well by us in the past, but I don't like to do anything that might be construed as advertising in the forum - freepmail me if you are interested.
Would it *not* be worth a faster processor for all that?