The board you linked us will work with both the Q6600 or E6750. Please note on the quad cores don't expect much of a overclock. You'll be lucky to hit 2.7Ghz with that board. There is a defect that has massive Vdrop and limits overclocking. That board would be great for the E6750 just not a quad. Also that board doesn't even support 1333FSB quad cores FYI. I had a battle royal with EVGA over it. I was pretty pissed and told them it was false advertising. They since have removed those claims from sites.
Go with the EVGA 680I A1 revision if you plan on getting the best clock out of a quad core. If you don't plan on overclocking beyond very mild that board will be fine for the Q6600 but not the QX6800.
I can't really quote on the bottlenecks to much since I don't have a ultra. I know my E6600 bottlenecked my 8800GTS till around 3.3Ghz then the card leveled off.
I'm not sure if your familiar with 3dMark or not. Its a benching system designed to see how well your hardware will run future games. Well a easy way to test to see if its bottlenecking the card is watch the graphics card scores. As you overclock your cpu you'll notice your graphics card score not the overall score go up. When the actual graphic scores stop going up you know your cpu is processing the card to its fullest potential.
I'd say pick up one 8800GTX and run it. If your not overally impressed with overall FPS pick up a second card. The only advantage purchasing a ultra would be your gauranteed the specific clocks. You most likely can get the same clock with the 8800GTX but its no gaurantee. So if you want to make sure both run at the same clocks maybe it would be wiser to just go with the ultras. That way say you get one 8800GTX that can overclock to 680core and the other only does 660 your limited to the 660 even though the other one can run faster. If you get pre overclocked cards you know there both going to at least run at a certain speed. Then theres the potential to overclock them some.
Be sure to stick around the forums that sounds like a pretty hardcore system