In general, the more 3D oriented, and state-of-the-art,
the less likely the card is to be supported. This is partially
a function of the proprietary nature of 'gamers' cards,
and partially the unlikelihood that researchers in Plan 9
have found the need to support such cards.

If you read the hardware compatibility specs, you will
see what is supported in the current release. The usual
line is 'ATi mach64 based cards are OK'. Of course, if
you see it mentioned in /lib/vgadb then this is encouraging.

Support is improving, though. If you check the archives,
you will see that a Voodoo 3 and S3 Savage drivers have been
done, but not yet released. Matrox G200/G400 may not be far away.

On the flip side, no Ati 128 bit cards are supported.

Another important rule of thumb is 'does XFree86 support
it'. This is an indication as to whether the technical
documentation needed to write a driver is available.
At one time certainly, Nvidia were difficult.