About the only video cards known to be Mac-flashable are nVidia GeForce chipset cards.

The others have the problem that there were NEVER Mac versions, so where would you get Mac ROMs for them? (Not to mention a complete lack of drivers.) The Winbond and S3 chipets for sure. The Matrox chipset might be the equivalent of a Millenium, but I wouldn't go mucking around. It's not like those chipsets are significantly more powerful than the onboard video of any PCI Mac.