PCIe Graphics Card for Pentium 4

Hello.

I am getting a new gaming system these days, but I think I will try to push my old one just a little bit further (maybe for lan sessions).

The old one is a Pentium 4 Prescott 3.2Ghz with 2Gb of DDR400 Ram (4x1). I plan to buy one of those p4 mobos that support pci-e in order to get a graphics card upgrade for it. Up to now, I had Gainward Bliss (GeForce) 7800GS+ AGP, which still performs pretty decent, considering for example that it handles source games (up to left 4 dead) on max settings. So, the question is:

What is the most powerful pci-e graphics card that a prescott can handle? One that will not be bottlenecked by lack of dual cores or anything else for that matter. It would be also helpful to know what kind of improvement such a card would be over that 7800GS+.

Thank you.

Thank you.