I've mixed Matrox and ATI, no problems. (ATI RagePro AGP as primary, two Matrox Millenium cards as secondaries.)
Theoretically, at least, you could even mix ATI and NV cards in one rig, but I would be certain to strictly disable all non-essential "enhancement" applications, like ATI's hotkey thing, and NV's desktop-management stuff. I've also mixed newer (multi-mon capable/MMIO-able) S3-based PCI cards with any other cards with no problems. (Might want to likewise not install the S3 refresh control-panel software though.)