As far as the CPU goes, you could try a 5400+ BE
if the motherboard is socket AM2. If it's 939 then the best you could do without upgrading the motherboard is probably a 4800+ (rated at 2.4GHz I think). As far as GPU, you might have to get a PSU with a PCI-e connector depending on a card. You could go with a 8800GTS
(which is at a very good price right now: $80 with promo code).
If you do buy the 5400+ BE you'll need an aftermarket cooler.