The E6600 is also a little outdated.
The E6750 is a cheaper, a little faster clock, slightly faster clock for clock, runs cooler, and overclocks much better.
The Q6600 is likely a little more future proof and probably the better CPU, but the E6750 is a very good deal as well.
The slightly higher clock of the E6750 may let some games run a bit faster that do not take advantage of all 4cores of the Q6600.
However, the games that use all 4cores, the Q6600 will have a very large advantage. More and more games will support all four cores as time goes on.
My vote is for the Q6600.
The E6750 if the saved cash can let you afford a better GPU.