When does the GPU start to Throttle?

I got my GTX570HD. At 30% fan speed its very quiet. But if I leave it at that while gaming my card gets to about 90 degrees. And while playing it seems like my card starts to get a little choppy while playing BC2. At what temp would the card start t throttle or slow down my game play? Because when I set it to auto the fan speed gets to about 60% and its way way to loud for my tastes. Also when gaming with a ATI card like the 6870 or 6950, it seem like it plays it smoother than the GTX570 I know that sounds crazy but it really seems like that. Is BC2 more optimized for ATI than Nvidia?

I believe they throttle at 100°C. 30% seems a little low for a constant speed. I'd use Afterburner to set a custom fan profile that turns the fan up gradually as the card goes over 80°C. So leave it at 30% until it hits 80°C, then have it go up to 50% when it hits 90°C. That might keep the fan quiet enough for you but keep temps a little bit lower for a happier card.

^ this.