The GT220 should be able to play games like L4D2 on high. Crysis on low-med and GTA4 on low. It kinda' depends on what size monitor you're playing on though..
If you wanted to upgrade your CPU I would suggest looking at the Intel i3 CPU's, though I would assume that would require a motherboard change.
If you're not looking to upgrade your motherboard as well, tell me what your board is and I'll see what options you have.