Go for the Q6600.
At stick speeds it will not be much slower than the E6750 (in single thread apps).
It will also overclock much better on your FSB weak platform with its 9x multiplier (the E6750 has an 8x multiplier).
In multi-threaded apps, the Q6600 will PWN the E6750 regardless of any speed disadvantages.
Not only does the E6750 have half the cores of the Q6600, it only has an 8x multiplier as compared to the Q6600's 9x.
This means that when you match the 333Mhz FSB with the Q6600, you will have a 3Ghz quad core.
Any way you count it, that will be quite a bit more powerful than a 2.66Ghz dual core.
As the 680i SLI is a very poor overclocker, it is highly probable you will run out of FSB headroom before you max out either CPU.
That will leave the CPU with the highest multiplier in the best position.