Unfortunately, I only have an AGP 2x slot and I play mostly simulation games.
No point in upgrading it in part.
I agree with Track on this, it's all or none.
Putting an X1650Pro or GF7600GS (the two at the edge of your price range) on that rig would be pointless, the memory and the CPU are going to hold you back.
The FX5500 is likely rendering all the rest of the system can throw to it. 256MB and those 2 P3-500s are killing you (few games would even take advantage of the dual procs with that little memory).