i think u r wrong, if the slot is 16x , then the card will run @ 16x
Not true. A PCIe 16x slot can have up to 16 lanes of PCIe bandwidth attached to it. It could have as few as 1, or the limit of 16. Motherboard manufacturers should put nothing but 16x slots on their motherboards, as any PCIe card would then work in any slot, and would simply use whatever bandwidth it had.
Have you never seen the Asrock boards that let you chose between running an AGP or PCIe card? When these first came out, they had a 16x slot, but had only four lanes attached to it. (again, if the linked article is the one I'm remembering, they used this board so they could measure 4x performance.)