Well, seeing as you can get a 1GHz athlon for under 100, and motherboards arent too expensive, and you can even get DDR for really really cheap right now, I would have to say that this is one of the best time to build ANY type of system. Since DDR is probably going to stick around for a while and you can get a DDR board for like 135 and a 1G tbird for under 100 and 256 pc2100 for like 65 bucks, that puts you at a whole new system for 300. You will probably want a new PSU though, I went from a celeron to a tbird and needed a new one.
If you arent looking to spend that much, you can just use your old RAM(if it is at least PC100 or PC133) and just buy a KT133A MB and stick it in there. They run just over 100, so that would put you at just over 200 for the MB and Proc.
Just my thoughts
Paul Hilgeman