Using ASUS mobo ,primary GPU will work @ x8 mode ,even if i install 2nd gpu on 2nd pci-e slot for physx , its that bad for games primary working @ x8 mode... info from : http://www.tomshardware.com/reviews/pcie-geforce-gtx-480-x16-x8-x4,2696-5.html
Not that I like Tom's but to me it looks like whatever they tested only 1 game(COD) saw a difference of any note which is curious.I personally don't see any difference of note at 8x vs 16x @1920x1080.
Just a heads up, there is another thread like this somewhere. And no, the difference between 8-16x is very minimal.