With 200w power supply i wonder how are you able to run even a Ati 9600xt.
The minimum requirement for you would be to change your power supply...
A 200w is nearly not enough for a card which can run games like GTA on mid-low details. Actually, the only card which is capable of doing so is Radeon Hd3850 agp.
But your 2.53 ghzz processor will definitely bottleneck it and so will you 4x agp slot.
So my reccomendation for you would be to completely change the system now. It is very old... for about 300$, you can get a Pc which is just uncomparable with yours.