somebody said to me your graphics cards work as x8 Is this correct? 
Sorry, had the wrong quote before.
What are the two 'non-graphics cards'? Are they PCIE-SSD cards?
Perhaps verify with 'Tech Support' regarding your intended SLI configuration, whether it will be different from the recommendation
of using the first PCIE-x16 & the third PCIE- x 16 slots on the board, as you intend to use two cards only.
The data PCIE cards should work without any problem in any of the other 5 empty slots?
I'm sure the system/board should work but I would get advice from tech support before set up!