Quadro 4000 for mac is the only 'official' Nvidia for Mac card - and is currently very good - but is about £600-£700

An alternative to the 'firmware adjusted' GTX285 and similar performance to the Quadro 4000 is the 'firmware adjusted' GTX480 - new on the 'firmware adjusted' market and about half the price of the Quadro 4000
GTX480 (PC version) is readily available.
GTX570 may also be a contender - TBC

(use at your own risk)

S