An 80W TEC is made to be used at 16V, at 12V it handles at about ~55W. A 172W Peltier is made to be run at 24V, at 12V (if my math is right) it comes to roughly ~66W.
An 80W TEC would probably work, the 6800 apparantly doesn't put out that much heat compared to older GPUs, like the R350, but your temperatures wouldn't be great, especially if the block is added to the rest of your loop, with a CPU block also adding to your water temperature, and further increasing your peltier's hot side temperature.
A more powerful peltier could also be a bad thing if your CPU and GPU are on the same loop, sharing water. A 172W peltier on ~20+ V puts out a *LOT* of heat on the hot side, and would uttely trash your CPU temperatures.
Having a TEC in your water circuit, with other un-TECed waterblocks, becomes a difficult and ultimately futile balancing game. The TEC hotside dumps heat into your water, which in turn ruins your CPU temperatures, which in turn heats up the water even more, which is bad for everything being cooled.
If you want to get the most out of a TEC-cooled GPU, you really need to have it on a seperate loop, independant of your CPU, and other waterblocks.
