The GPU is always the bottleneck if you push graphics details high enough. If you do not push the GPU that hard then which of the two is the bottleneck will vary from game to game and even from scene to scene within the same game.
The CPU is rarely a problem unless you have something low-end or run unusually CPU-intensive games such as 64-players BF3 maps or use FRAPS to record games.
I mean, I mostly play Arma 2 (and 3 when it gets released), and the soon to come out GW2. I just want to make sure all my bases are covered and I don't get screwed because I bought something and it wouldn't work to it's full due to my hardware.