The Radeon HD 4650 AGP would be near (if not the actual) pinnacle of performance you can plug into your old system as long as you have a power supply capable of supporting it.
The real question, however, is how much money do you want to spend on this aging system? In all honesty, you may want to live with what you have for now and save your money for a more modern system. PC prices are almost ridiculously low when you consider the performance you can get for your $$$ these days.
If you want a cheap brand name system, you can go to Dell's Outlet (dell.com/outlet) and buy an awful lot of PC for little money and it will be warrantied.
If you want to built your own, establish a budget and run that budget by the collective expertise of Tom's Hardware and the greater geekdom will help you design a true beast for your money.
AGP really is a dated (as in dead) technology that served its purpose well in its day. That really isn't today.