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.