Fry's ad today had an X2 4200+ and motherboard for $85. Also had 2gbs of Kingston HyperX for $50.
I haven't tried this before, but here is a wishlist I created in newegg, if you can see it it worked:
Its an x2 4000+ cpu, motherboard, 2gbs of ram, case and psu for $250
Its microatx which some people hate. I happen to like them. Smaller PC. You can usually only fit 2 cd drives in them...but who needs more than 2 cd drives? Alot of prebuilt machines are microatx, actually. If its a full tower...its very tall.
Don't buy parts based only on my reccomendation.
Alot of people here know much, much more about this kind of stuff than me.