The advice I received from a computer repair/upgrade place about this question was:
1) Affordable upgrade cards that will work with this machine and software are almost unavailable new and may take up to 6 months to come.
2) Upgrade cards that are readily available and will work with this machine and software are unreasonably expensive.
3) The existing ATI Radeon 9600 Pro card will work, if slowly, and I should just use that until I can upgrade to a new machine.
That isn’t really what I wanted to hear, and it contradicts what Apple told me about the requirements of the software, but the salesperson sounded very familiar with the problem.