OC'ing was a breeze, absolutely no issues at all. I was able to overclock my CPU to 4.5ghz with no bump in voltage. To get 4.6 I had to bump the voltage up a little. So I decided to keep the lower voltage with the 4.5ghz clocks. Im sure I can get it to 4.8 with a higher voltage too but I want to keep things stable and under normal temps. Memory is stable on the standard 3000mhz frequency, just had to enable XMP profile in the bios, but anything higher then 2133/2166mhz? memory is considered overclocked. I chose only 1 GPU because the 980TI is amazing, and Ive seen benchmarks that didnt really impress me with the improvements of FPS in SLI to warrant another $600+ right now. Plus I didnt want the headaches of multiGPU support/optimization. Maybe if DX12 and developers will allow stackable performance in SLI, one day I will. My GPU is overclocked 150mhz core and 400mhz memory. I am absolutely impressed with my current build and no games/software can shake a stick at it.
Ps. Dont for get to grab a CPU cooler as The 6700k doesnt come with one and I didnt see one on your list. I also hope you will be running ATLEAST a 1440p display going sli. Otherwise 1080p would just be silly.