Need info on Video/Graphics card upgrade


I am wanting to upgrade the video card on my Mac Pro, but I don't know what card is correct for what I have.

My Mac is a MacPro 2,1, March 2008, Quad Core Intel Xeon, 3Ghz

1.33 GHZ bus speed, 667 Mhz DDR2 FB-DIMM memory

It is running 10.5.8 OS

Currently, it has a Nvidia GeForce 7300 GT that has only 256 MB VRAM

I would like to have as much as I can get for 3d work, but I think 512 is as much as I can get for this model?

Any advice on this would be greatly appreciated.

Thank you.