David -
Here's a recent post from my blog, which will help you through the process of getting an "off-list" card up and running:
http://bourkemedia.com/cs6-nvidia-card/
It's not difficult, but a matter of choosing a card which has the horsepower to run the Mercury Playback Engine, as well as the new 3D feature in AE CS6, which requires a bit of horsepower as well. That's a separate text file edit. Check out the links in my post - there's lots of information there. Good luck with it!
Joe Bourke
Owner/Creative Director
Bourke Media
http://www.bourkemedia.com