Radeon 7850 drivers for 10.8.3?

I have a Sapphire HD 7850 that I'd love to put into my early-2008 Mac Pro (my 3870 runs hot driving my 2560x1440 monitor). I'd heard that there were Radeon 78** drivers in 10.8.3, but I'm not having any luck with "plug-and-pray."

I plugged in the card, and it shows up as a "GPU" in the System Report, but doesn't load any drivers.

Any advice?