Best low profile energy efficient Graphics card

Hi,
I need now to choose a Graphics card Im trying to keep size to a minimum but will sacrifice if neccasary (length isnt a problem id prefer to keep height to a minimum though).

450w psu max (buying SST-ST45SF-G 450w as soon as it comes out here currently running 300w sfx size psu don't intend to use gpu until 450w is installed last mobo didnt like the strain)

Intel Core i7 3770 LGA1155 CPU 3.4Ghz 8Mb Cache Ivy Bridge
ASRock H77M-ITX/H77/2 x DDR3/1 x PCI-E3.0
Corsair H40
OCZ 256G Vertex 4
G Skill 16G(2x8G) DDR3 1600MHZ
???????????????

Any idea's?