Upgrade processor if anything, that 2.20 has a 400MHz FSB theres your bottleneck. See if your mobo will support a 533MHz or 800MHz FSB chip. If your mobo can and you don't want to spend alot of money go with a Celeron D 320(or higher) with 533MHz FSB. Newegg.com has Celeron D 320 for $71.00 then OC it to what ever you like. Celerons are good OCers and they do very well even without water cooling. Mine does 3.8GHZ on air, I have a Celeron D 320.
I have a ASUS 9550 GE(ATI) OCed at 305MHz core and
500MHz memory and it can pull 2900+ on 3D Mark 03 with my Celeron at 3.8GHz.