Well this would be an upgrade, and it would be faster than what you have, and its only $35.http://www.buy.com/prod/intel-pentium-d-820-2-8ghz-800mhz-2x1mb-socket-775-dual-core-cpu/q/sellerid/14981974/loc/101/210402962.html
Intel Pentium D 820 2.8GHz 800MHz 2x1MB Socket 775 Dual-Core CPU - PD820280775-R
This is better but its used, around the same pricehttp://www.buy.com/listing/sellerlistings.asp?sku=216376948&buy=1
Intel Pentium D 930 3.0GHz 800MHz 2x2MB Socket 775 Dual-Core CPU
It did come out 6 years ago, and your system is pretty outdated at this point so you might just want to start all over, but if you're going on the cheap this is pretty much the only chip that would make sense.