As mentioned your other parts will hold you back even with the best of (old-school) PCI cards. That being said, it is unlikely that even a high-end PCI card will have the power to run (more current) games at a decent speed. Not to poo poo anything, but it's way past time to investigate building (or buying) a new system.
If you must
try sticking with PCI I'd say try something like this Evga 9400 GT
at $70 post rebate. I can say though that with those specs you are probably going to struggle quite a bit even at lower res with current games. But, at least with Evga you'd have the 90-day step-up policy should you decide to go ahead and upgrade the entire system. You could then send in the 9400 GT to Evga and trade up to something vastly more powerful, like the 9800 GT
for only about +$30.