I would also say that some custom designed cards with back plates may actually physically take up more room than 2 slots and interfere with the next card, they may not fit, the back plate on one may be up against the cooling fans of the card above it. Make sure you do your research on this. Reference model cards will fit without question. DCUII or ROG models may not. I can say for sure that 290X DCUII and 290X Matrix Platinum models do not, not fit. A socket 2011 is a must for 4 way, there are not enough pcie lanes on socket 1150, I believe sli requires a minimum of pcie X8 per card, AMD requires X4 per card for crossfire. as HiVizMan states this will also require a huge amount of power, 1 1500 watt power supply minimum but more than likely 2 1200 to 1500 watt. you can find power supply calculators with Google.