GTX570 is overkill for 720p. One of the new GTX650ti's should be fine. Or if you went AMD there is the 7770, 6850, 6870, all of those would give you great performance at that resolution. You would be spending $100-$200.
Here is the 650ti, dual fan model, should run quiet and cool. $160http://www.newegg.com/Product/Product.aspx?Item=N82E16814121669
XFX DoubleD 6870 $170http://www.newegg.com/Product/Product.aspx?Item=N82E16814150521
I wouldn't get anything better than those for 720p. I run a 6670 on my HTPC and it plays all my games @ 720p with high/ultra settings very smooth at 60FPS. The 6670 is a lot weaker than those cards I recommened.
For $130 this is a good card too:http://www.newegg.com/Product/Product.aspx?Item=N82E16814102967