Well if you change the motherboard & CPU you'll more then likely need to change the RAM.

You need to pick AMD or Intel - big can of worms.
Do plenty of research, pick a motherboard with good momory bandwidth and a good CPU future ahead of it - so you don't have to replace it for a while. Then pop in the lowest powered CPU it can take (save money) and a fair chunk of RAM.
Something like a P4 1.6ghz (not the orignal one) on a non RAMBUS/PC133 board with about 256RAM (more if you run XP).
If you go AMD a Duron 1gig (better core then the 900) is a good starting point as the motherboard will also accept their flagship Athlons.

Sorry I can't point you towards a specific setup - but azzo.com have some solid CPU & motherboard combos.

Other option might be maxing out your current motherboard CPU wise and using faster memory (CAS2) - but you'll need to post more info for that to be discussed.