pci graphics
Ok, the only place I know of that you can still get a 5600 256mb pci is
here . $150. The 5600 beats the 5700le. The 5700le is 250mhz core, 400mhz memory, and 4 pixel pipes. The 5600 is 325 core, 550mhz memory(I'm not sure how fast the memory is on the 5600 I listed, the site doesn't specify the speed, but some mfg's do use slower memory), and 4 pixel pipes. So the 5600 is 75mhz faster for the core, and 150mhz faster on the memory. They both are 4 pixel pipe gpus.
Notes:
5600 325mhz core, 550mhz memory.
5700le 250mhz core, 400mhz memory.
Pci slots don't have an "x" rating like agp, pci is pci, unless it's pci express(which you don't have, so don't worry about it).
There was a fx5900 256mb pci at one time, but I think it never made it past the prototype stage. I think that would be the best pci card, but it isn't available. The 5600 is still available, but probably not for long, as I can only find it at one store.
Edit: The inno3d agp 8x version of the 5600 has 550mhz memory, so the pci, I would think would be the same speed.