Mudbox is certified with Quadro cards or firePro cards.
Pick up a quadro. The higher the number the better. Depends on your budget. The 4000 costs $800 and would do a heck of a job for you, but if you're looking at $600, you could get the 2000 at $450. It's still a certified card with mudbox.
If you can save a little more for the 4000, I'd do it.