Pairing a Radeon card with an SLI only board? Do you have the card in the top or the bottom slot? Try switching slots. I wondering if you just have it in the 8x only slot and your board isn't switching to 8x/8x mode because it's not detecting a card in the 16x slot.
The 750a chipset only supports SLI. You cannot add a second 6850 latter on to run in CF. There is a driver hack for this that involves using the drivers for Dells older XPS desktops using the 680i chipset. It apparently removes something from the nForce drivers that allows you to run CF on that board but I have never used it myself so I wouldn't be able to tell you much more about it.
As for the slots I think the first one on that board is suppose to be 16x. If your card works as 8x in the second slot then it suggests somethings wrong with the top slot and you may want to RMA the board. If it still comes out as 4x then it could be the video card, but you'd have to try it in a computer with a perfectly working 16x slot to be sure
I thought the same thing when I buy this mobo but They told me that there would be no problems, bad idea xD
Yesterday disarmament whole team did a thorough cleaning, try to put it into the second slot but could not enter, then resignedly put it in the first again, so i went up in there were some changes, booted faster, and now I read it x8 a great relief not to have been made, may not fit either the VGA or maybe the RAM, and thank you for this great help you gave me uan, a hug to all =)