dude it wont be like, easy to oc a 6850 that far. you will need serious cooling for that, on top of some pretty sharp overclocking ability...not to mention luck lol.
i play COD4, BF2, ETQW, COH, FEAR with my 3600 and an 8800GT and have EXCELENT framerates. and that goes if i have it clocked at 2.6ghz or 3.1ghz.
you will not notice a difference in a q6600 @ 3-3.4ghz vs a 6850 @ 4ghz.
you wont have a noticeable difference between a q6600 @ 3ghz and a 6400. ie you wont be able to look at the game during play and say, "wow...i can tell a difference between the q6600 and the 6400" if they were side by side. you probably couldn't see any difference between my 3600 @3ghz and a q6600 @ 3ghz...playing a game side by side. benchmarks, maybe a small difference between the 6400 and q6600, bigger difference between my 3600 and a q6600, obviously. my point is, framerates are usually more dependent on the quality of your video card, not the extreme level of OC on your cpu. any dual core cpu clocked over about 2.5ghz (if the pci bus frequency is left stock at 2500mhz) will have relatively equal framerates. relative being you arent going to sit there and actually SEE the difference.
there is no noticeable difference between 70FPS and 95FPS, ya know?