when i had a 8800gt i had to up the freq from 100 to 110 to get it to give me 16x if not it would stay at 1x but if i would lower my fsb it would go back up to 16x so im guessing when you go for high fsb's you need to up that freq depending on your card

check gpu-z and make sure your card is running at 16x if not up the freq and it should go up to 16x