Running crossfire with a pci-e X8

I was looking at buying a motherboard for a new build I am doing. I plan to buy a 4870HD to use in the midrange build. I plan on using this card until it is not sufficient to run games, then using crossfire to add a second card.

But when looking at the motherboard it support one X16 slot, or two X8 slots ( http://www.newegg.com/Product/Product.aspx?Item=N82E168... ). When I eventually add another card for crossfire, will this slow down the speed of both cards? Keep in mind budget is tight, so I know it is ideal to have two X16, but how much will 2 X8 matter?