Thats is a "old model", if you have it for quite a while, let's say, more than 2 or 3 years, you may expect that it has given all it could to run your games swell, but now it is the time for it to rest.
I bought a AGP Radeon HD 3850 512 DDR3, if I remember right. It wasn't that expensive, it indeed gave me some FPS gain, I must say it "lifted" my system up. But still, there is my Athlon XP +3000, nicknamed bottleneck, and my dear pairs of 1GB DDR 400. Been thinking about getting, temporally, a AsRock, as it has support to AGP, DDR2 and dual cores. Maybe extend even further my system until I raise the sum to pwn any Allienware on the market. *Muses*