IF only for Gaming then of those two the i5-3570K. Gaming is mostly about the GPU(s) and sufficient RAM (8GB or more DDR3-1600 CAS 8 or 9), and as long as the CPU doesn't bottleneck the GPU(s) you won't see a difference. For the cost difference it's a no brainer to purchase a $100 better GPU.
750W is plenty and 850W is fine and will power SLI GTX 680 -- though I would recommend 900W with 100%/100% + 30% Aging. My assumptions are always: 2xHDD, 1xSSD, H100 HSF or its equivalent, 6x120mm fans, high CPU & GPU OC, 100% utilization and 30% capacitor aging.
You can't oversize a PSU and it sucks when you under-size a PSU (replace).