They're cheap enough as it is mate, get yourself a cheap E2200 or something, or E5200, they're pretty much the same price, and you will GUARANTEED see a massive improvement to your current PC. Get it with a P43 motherboard, a 2x1GB DDR2-800 kit and you're set.
This will require a reinstall of Windows by the way

Make sure the PSU is up to it, if not.. get a Hiper 530w, that is more than ample for your needs