I dont know how much money you have, or what you are willing to spend, or if you were already considering a 8800 gtx. BUT
The e6600 has about the same upper limits as the 6800 in overclocking, approx 3.6-4 ghz depending on if you are going to use watercooling.
If you were already considering an 8800 gtx, then consider saving the money by getting the e6600 and buy a second 8800 gtx for sli.
If you weren't considering a 8800 gtx, I would suggest getting it because it has more impact on games than 2-400 mhz worth over overclocking you could get outta the 6800 as compared with the 6600. Either way a core 2 duo running at 3.6 ghz should be plenty enough for gaming keeping in mind you the video card has the most impact.