I wouldn't bet on it. If someone were to "hack" or program a driver for one of the high end video cards why have they not done so already? The drivers have less to do with the CPU than which OS you are running. In all likelyhood a well written driver for a video card now will probably re-compile with very little tweaking for OS X on Intel. I've been researching writing an hardware driver for OS X off and on for awhile now. Drivers involve many calls to the kernel which should remain the same regardless of the underlying hardware.
In theory any AGP graphics card out there right now, even the high end ones for Windows should work just fine on Mac if someone were to write the driver.