Ive seen other threads with the same question. i see you got the a8n sli dlx mobo, same as me, is your sli card on the board between the pci x slots on single video card? Cause i know that the pci x slot are 16x but when you run an sli setup it drops the slots down to 8x, so that both of them combined will hit 16x. Other then that try load the old drivers and see what it says.
my situation was that i am running the 7950gx2 card, that card in general is an sli card so my board automatically thinks i am running an sli setup and drops the slot down to 8x. So i picked up a a8n32 sli dlx yesterday on clearance from my local pc shop for 145.00. On that board it runs full 16x on single or sli setups.