Nvidia 8800 GT (mac version) is really your only choice. Don't buy a PC card - it wont work. You need the Mac version.
Ive just tested a DH2G-Digital connected to an 8800 and managed to get 3 x 1920x1200. 1920x1200@60Hz from output 1 and 3840x1200 @57Hz from the DH2G-D (see screen shot attached). 1920x1080 doesnt seem possible via DH2G-D
If you dont need the 'programming' screen - then you could use the 2 outputs directly from the graphics card and ignore the DH2G
Simon