A Digital Audio Powermac ought to be able to use the same video cards as later Powermacs.
Reflashing a PC 9200 or 9250 would probably be the cheapesyest solution (using a 128MB model as OSX and MOS don't support more than this on those cards).
Of course this process can be intimidating.
First, its easier to do the reflashing on a PC (where the card will boot correctly).
Then, you need to move (or remove) one surface mount resistor (with a soldering iron).
And, finally you have to mask over or cut two leads on the card edge.
As mentioned before, the instructions for all this are on the internet.
It sounds scary, but I replaced a failing 64 MB 9000 with a 9200 in under a hour at less than $20 cost.
And, while the specs of the 9000Pro (or better yet the 8500) should lead to faster performance, in the real world I don't see any difference and the extra memory is very useful under MOS.