You are not bottlenecked. There are people who think that a Phenom II x6 overclocked to 4GHz is not enough. These people are crazy.
I don't have experience with crossfire 5770s, so for all I know you could get a couple more FPS with an increase in the CPU frequency but... that's certainly not "bottlenecking". The term "Bottlenecking" is overused and misused. I have an athlon II x4 620 overclocked to 3.25GHz on stock cooling and voltage and would have no problems putting in a 5870 or more (I'm not fond of the crossfire idea).
You would still benefit more from a 3rd 5770 more than you would by a new CPU for sure.
I have a radeon 3870 and an Athlon II x4. Dragon age Origins is CPU intensive. But upgrading briefly to a GTX295 increased my framerate dramatically.
There are a few exceptions. I haven't played GTA4 but I have heard it is horribly coded and will benefit much more from a better CPU. And of course there is the game Flight Simulator X, where I heard it is so CPU intensive it would be better to sell one of your graphics cards and get a phenom IIx6 1090t in order to get higher FPS. But these are rare.
Ask yourself also: Am I happy with my framerate or not? If not, you will need a whole new computer to get it up enough.