You probably won't get the 6600GT faster than 525/1050 on stock.
Also, it's simply outdated, the whole 6600 series is. The 6800XT and 6800GS are barely staying alive, but will be killed off within the coming months. The 7300, 7600 and 7900 series are the three areas you want to look at for a new video card, but for gaming I'd look at the 7600 at the lowest.