As regards to a mobo & cpu combo to upgrade to it all depends on two things
a) your budget
b) what you want to do with your pc
If your looking to do a lot of media related things like mp3 encpoding, and video stream encoding, go with a pentium 4 solution, prorbably for speed and power go with a pentium4 M , which is based on the much better pentium 3 architecture
If your a game nut either go with a 64bit sempron, or they fully fledged amd64 fx etc
if your looking to do it on a budget, get a socket a mbob with all the latest features and get either a 2500+ barton core(just up the fsb 400 and hey presto 3200+ for much less
or if you can get one at the moment( i know i couldnt recently got a 3200+ xp
hope this helps
