Barring any major overclocking of the CPU in the system and a ton of other 12V-dependent devices, the best performing low-power and low-profile video card is the ATI 5570, followed by the 4650.
There are 9800GT and 9600GT low-profile cards available, and they would outperform these cards, but they also require the use of a PCIe 6-pin power cable, which most sub-300W power supplies lack.
My personal recommendation would be the HD 5570 or 4650, depending on your budget. If you're in the US, Newegg.com and many other online PC component e-tailers sell them.
These are Newegg's 5570's. Mind you, only 6 of the 9 are low-profile. (The XFX, Gigabyte, and bottom PowerColor are not)http://www.newegg.com/Product/Prod
And here is a low-profile 4650.http://www.newegg.com/Product/Prod