Grab a PCIx16 flexible extender.  I currently have four moderately high overclocked cards running on my MSI 890fx-GD70.  Lol my cooling solution is to leave the rig in front of a small AC unit, temps are always under 72C.  Oh and the cards I'm using are a 6970 thats running @ 420mhash, a non reference locked shader 6950 @ 370 mhash, and 2 5850s @ 350 mhash a piece.

If a small AC unit is not a viable options, get a bunch of case fans! Keep your cards under 80C!