No offense, but your PC is more or less obsolete by any current standards. CPU is far too underpowered, and GPU is no longer supported. 640MB RAM isn't enough to run most apps without noticable slowdown either.
In short, its probably less of a headache to junk the PC and start over.
The problem is the PSU. You don't have enough power/current to run a modern GPU (or even CPU?), and a CPU upgrade is a must before any GPU upgrade has any effect. In short: You need a new CPU, GPU, and PSU, and possibly a new mobo if newer parts aren't supported. Plus the fact AGP GPU's are overpriced due to lack of demand.
It'll cost around $500+ getting that rig to a point thats ok, but still old, where you can get a decent modern PC for around $650 or so. It doesn't make much economic sense unless you can dig up some major deals.