The solution that i came up with, considering I have the same dilemma (though not yet implemented), would be having 2 single slot cards which incidentally comes in the form of the AMD HD79XX series + Swiftech's Komodo Series (gives you a free single slot i/o plate and vga backplate for free) IF you are opting for a custom liquid cooling setup. Your best bet for air would be Afox's single slot AMD HD7850 which is downright sexy (not as sexy as XFX's HD7750 single slot edition though) and benchmarked quite well with impressive review(s) but good luck with finding that card.
My rig: i5 3470K@4.2, Asus Maximus V Gene, Gainward GTX670 Phantom, Asus Xonar Phoebus (beats the hell out of the mobo's built-in audio), corsair vengeance 16gb, corsair ax650, crucial m4 120gb, 2x 1TB HDD, Silverstone FT03B, Corsair H80i.