PCI-X is just extra-fast PCI, and is backwards compatible, unlike PCI-Express. As such, you should be able to put any number of Mac video cards in, so long as they're PCI. (EG Radeon 9200, 7500, etc). AFAIK the best one is the Radeon 9200 PC/Mac edition (one card works on both systems; it shouldn't be to costly/hard to find). If all you need to do is drive a display for simple 2D stuff though, an old 7500 etc will be fine though.