Before you upgrade:
1. Run a defragmentation (Primary cause of old PC's becoming slow)
2. MAX out your system's RAM (4GB is the max for your motherboard [1GB modules in each slot])
3. Buy the CPUhttps://www.upgradebay.com/c1_itemdetail.asp?rid=22&itemid=168715286
(and that is the best price I could find.)
I don't reccomend you spend that much on a CPU though.
You would be better off spending money to buy a new motherboard, CPU and RAM.
You could have a Quad Core CPU with a faster FSB and 8GB of RAM for LESS!!