The 6800 GT or Ultra are awesome cards for gaming, however with the new Core Image/Video features of Tiger they will also be powering even more of those real-time effects which make video editing much easier. For the moment they difference may not be hugely noticeable, but if you can afford it then you will see the benefits as more and more of the processing is offloaded onto the GPUs, and the 6800 is one hell of a powerful one.
A consideration that may be important for you, if you have, or intend to have a RAID array for storing video (they are necessary for capturing HD video in real-time, and greatly improve loading time), then the fact that the 6800's take up both the AGP slot (slot 1) and one of the 100mhz PCI slots (slot 2) might be a consideration for you as PCI slots can be at a premium for video-editing as you start wanting high quality video input cards etc.