I am pretty sure that the best card for OS 9 and OS X together is still the GF 4Ti.

Also, there is no card that supports core image in OS X as well as being old enough for OS 9 drivers. Just so you know. (but you can still get Quartz Extreme&OS 9 support with one card)

You best option is to have a dedi OS 9 machine and a dedi OS X machine if you really want GFX power in both operating systems.