no, that board can't support duel 16x lanes, check the jumper card on the board and make sure it's set to the single card side.
don't worry though, even if your only running at 8x, the 8800gts can't saturate it anyhow. so your FPS or 3dmark scores are not being hurt. i battled with the p5n-e for a long time and found 8x or 16x - the scores are the same.