bottlenecking simply means you are not using your video card to its full potential, and will be held back by other components like cpu. to simplify things a bit, your system is only as good as the weakest component in it. the relaitve importance of a graphics card or CPU will depend greatly on the game. In your case, a 6850 may be your bottleneck in some cases, while the q6600 will bottleneck you in other cases. I dont know what your current graphcis card is, but getting a 6850 is not at all a bad idea, and I would encourage you to do it if you have a sufficiently powerful PSU. the q6600 is still a decent processor, and most games are GPU dependent at your resolution anyway.
As for getting a 6770, what he simply meant was that you would be saving money by getting a 6770 since you may not benefit from a 6850 due to it bottlenecking you. Having a CPU bottleneck will not give you hiccup or lags. I would disregard his advice. A 6850 would make a decent match with your processor, and you would definitely get an improvement over a 6770.
Stick with the 6850, its actually quite a good balance for your resolution and the majority of todays games. Yes its a pain in the butt that you cant Over clock it but to be honest its not going to be any kind of issue that you will actually notice.