Sometimes driver updates can decrease performance somewhat. The problem with nForce drivers is that it's not just a graphics driver, it contains drivers for the chipset which can affect performance of the entire PC.
Whatever you are using to get your graphics score is probably based on synthetic tests. The best thing to do is to actually play some games to determine if there is really a drop in performance. If the game has a built-in benchmark mode then use that and compare the score prior to updating nForce.