Crysis uses one thread for itself, and one for graphics calls. It will pitch an absolute fit with only one core available, but gains nothing beyond two. So high single-thread speed is essential.
If you want to see exactly how far back the CPU is holding you, set the game to a very low resolution, but with all the detail settings at max, then run the included GPU benchmark (benchmark_GPU.bat in the binaries directory).
Keep in mind that different aspect ratios have different amounts of rendered objects in the field of view, so the results are only valid for the same aspect ratio.
For example, I just did a couple benchmarks with my 3930K at 4.6GHz. For the 800x600 run, the min/max/avg is 65/136/99. That's the best my CPU can do at 4:3 aspect ratios. At 1280x800, the numbers are 61/127/92, which shows that the increased FOV with a 16:10 aspect ratio puts more load on the CPU.
When I raise the resolution to 2560x1600 and add 4xAA, my two 7970's at 1200/1450 each get 50/83/68. Normally, with vsync on, this means I get a steady 60fps.
So it's possible to get good performance, but you need a fast CPU.
As for moving the card, according to the manual I just downloaded, slots 1, 3, and 5 are x16, so you should be fine there.