You should be good at 4.0Ghz or higher. Even stock is fine. What cards do you have in tri-sli? 670's or 680's? If so, you will likely see some bottlenecking of the CPU, but mostly only because you are getting more FPS than your monitor can display. Two 670's or 680's should be plenty to power that monitor size with a 60hz refresh rate. Though the 3rd will allow for closer to max settings on the rare game with settings generally unplayable by normal systems, like Metro 2033 with advanced DoF, or The Witcher 2 with Ubersampling (still not likely able to use that).
Edit: doh, I missed the mention of 570's right at the beginning (or forgot). Anyways, your biggest issue is going to be memory. 1.25Gb of vram is likely going to be the main limiter in games like BF3. You may be best off just sticking with 2 if it's not too late, and just upgrading completely later on. As you push higher and higher on the GPU power, more vram becomes needed.
So, geofelt, do you still believe I'll see any performance and/or bottleneck with GTX570x3 @ 2560x1600 with an i7 2600k/4.5ghz? I can push the clock higher but my temps are around 66 under load and that's usually my limit.
My guess is that the third GTX570 will look good in benchmarks, but you will not feel much difference in gaming.
Similarly, I don't think a higher overclock will be felt either.
You might try a test with your games, and reduce the OC from 4.5 and see if you can tell any difference in actual gameplay.
If you see any reduction in fps, then you might assume that the cpu is a limiting factor, and that a faster chip or higher oc will do you some good.
It is hard enough to get two disparate graphics cards to render their parts in sync, let alone three.
I wonder if you might be subject to microstuttering, even if you have three high end cards.
You might want to read this article on microstuttering: http://www.tomshardware.com/reviews/radeon-geforce-stut...