I believe Harris has a graphic card compatibility list for VelocityHD. I'd opt for a card that's a bit older with very well established and stable drivers vs. something brand new.
I know almost all the Nvidia line from 2-3 years ago were virtually all compatible. But just google "graphic card compatibility" for VelocityHD and I'll bet you can find a certified list.
Chris Blair
Magnetic Image, Inc.
Evansville, IN
http://www.videomi.com