If you can power it up, just burn MorphOS ISO to the CD and try to boot from it to see what happens. Press C key on startup or select from alt-menu to boot from CD.
Graphics card must be with Mac BIOS, so standard PC cards won't work even if they're listed as supported models.
If you have to get new gfx card, I'd recommend you to get R8xxx or R9xxx serie models. R7500 has reported to have some slowness issue on PowerMacs by some people. For example R9000pro is a good choise as said. And more the gfx memory, the better.
