You may run into some driver problems, so I personally wouldn't do it. One of my friends tried to do this a while back, with a Radeon 7500 64MB as his primary (AGP) and a GeForce2 MX 200 32MB (PCI, duh 😛 ) as his secondary. Granted, this was a while ago and these issues may have been fixed. The ATI drivers refused to play nice with the NVIDIA ones, and there, at the time, was no way to fix it. He ended up selling his MX200 and getting a PCI R7000, but it was a headache at the leaste. If your feeling brave, then do it, otherwise, pick up a PCI card from ATI.