Is that your only processor choice because i can recommend
Processor AMD 3700+
Motherboard (assuming you won't be overclocking heavily)
MSI K8N Neo4 Platinum (nforce 4 ultra chipset)
OR for slightly cheaper (even though the processor is $50 cheaper than the pentium
MSI K8N Neo4-F (nforce 4 chipset)
of the two i would reccommend the platinum because it has a better chipset, i believe that the ultra has a higher bandwidth pci-e slot but i could be wrong, but if that is too pricey then the Neo4-F is fine.
ASUS Geforce 6800gt 256mb
on the ASUS card i hear that the ram sinks fall off easily and that concerned me but if you are careful that should not happen. If that does concern you then this one would put you over budget by a little.
eVGA Geforce 7800gt 256mb
$275.00 after $20 rebate
This eVGA card is much, much better than the 6800gt so if you could spare the extra 25 bucks i would pick this one up as long as it is this cheap.
hopefully that helped, i didn't really explain a whole lot, just gave you some hardware options on AMD's side
the total budget comes out to be anywhere from $538-$620