PSU Calculator -> http://www.thermaltake.outervision.com/
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).