The ATI/AMD HD 3800 series are very good performers for the price. They also sport onboard HD (Blu-ray & HD-DVD) decoding with support for 1080p on 30" dual link displays (2560x1600). I have tested this on the PC side. This includes HDMI support with in-band sound.
The 8800GT (nv92) based cards have slightly faster 3D performance and have overcome the 7-series limitation of HD over dual link displays.
Both support DirectX10.1 but that is pointless on the Mac. One can hope that the Mac Pro rev will allow for Crossfire or SLI to increase performance. HP has shown that a single system can support both technologies (see their Blackbird 002 gaming system). Also, Diamond Multimedia demonstrated Crossfire cards running on SLI mainboards by using a driver virtualization technique. There is a sight (5%) performance penalty for this configuration, but it does allow for ATI cards to be used in performance configurations on SLI systems.
And in GPGPU applications the ATI cards are faster.