I would get a pentium 4, at least 2.8ghz, 800mhzFSB
Compatiable mobo (ASUS)
at least 512 DDR memory (3200)
i think you can use the rest of your old machine and hook it up to your new mobo, but i don't know what types of drives you have and if your graphics card is a PCI or a AGP and what speed (4x or 8x).
another question, what do you plan to do with this upgrade, what do you want the computer to accomplish? is it just sluggish and you want it faster? do you want to play graphic intense games?
the other people on this forum are a lot better at giving specs and prices and advice and such. but those are my two cents.