I have the ASUS GTX 550 ti (
http://www.asus.com/Graphics_Cards/NVIDI…50_Ti_DCDI1GD5/). Fits all your criteria as well. (I've always preferred nVidia cards over ATI, in part because I run Linux, partially because I've had issues with ATI cards in the past, but that's me).
This card is unbelievably quiet. To the point where when I first installed it, I had to check if the card was plugged in properly. See link above for other features.
The card is huge though. It takes up two slots at the back of your PC and so may block access to another expansion slot on the motherboard. It is also a bit power-hungry. ASUS recommends using a 500w or more power supply with this thing (though some have been successful with only a 400w power supply). Your 550w power supply should be adequate.
If you aren't big on ASUS products, the nVidia GTX 550 ti is available from multiple manufacturers including EVGA, Gigabyte, MSI and many others.