the core clock on the x800xt is 500, while the 6800 is 400..
but of course neither of these cards have mac os 9 driver support.
It is not a matter of pure memory or core speed, the driver support always makes the difference.
Some years ago I purchased for my collection a G5 dual core machine, I believed that plugging inside a Nvidia 7800GT would resolved all my "needs for speed" related to a PPC machine. I were wrong because while on the 2D and 3D side the performance were really good the HD video decoding was a disaster and reading some posts around about the most powerful Gfx card available, the Quadro FX 4500, the news were almost the same: that incredible video card didn't do nothing for accelerate the video decoding, only the CPU power was involved but always insufficient for a smooth playback.
Unfortunately I haven't that machine no more but I am sure that plugging a X1900GT inside will result in a GLOBAL acceleration on 2D, 3D and Video simply because its drivers take care (if the codec is compliant) of video decoding too.
I say that because I have a X800XT inside my G4 MDD and it can easily playback 720p files in H264 or a 1080p in MPEG2 while if I try to open the same files on a 4600Ti (that I also have) the screen remains black, if would been only a matter of memory or core speed I should see at least a choppy video but not a black screen, if I remember well a Radeon 9000 also performed better than a nVidia 4600Ti (limited to video decoding, we must be clear on this point) but not at the same level of smoothness of the X800XT.
This happened on the OSX side, on the OS9 side we haven't a native support for the 6800 or the X800XT but I bet that ATI's more advanced support in video decoding will show on supported cards some interesting differences, in short: while we all agree of superior performances of 4600Ti over all the other OS9 gfx cards in 2D and 3D I believe that video decoding will be better when seen on an ATI board.
My goal is to verify if this is true comparing the performances in video playback between a 4600Ti and a 9000Pro using various encoded material I will prepare for this purpose.
Stay tuned! :-)