I beleive thats a 915g chipset system which only support pentium 4 cpu's. So you might be able to find a P4-670 or 672 (3.8ghz) and run that. I wasnt able to verify that your motherbd supports those cpu's ( the chipset does) so please google for it before you buy one and definitely update your bios before the upgrade.
Personally, I wouldnt bother upgrading that old system. your money is better spent in upgrading the motherbd, cpu, & ram to something much newer.