Good question
When it comes to dual fan cards which circulate air internally I make sure the motherboard has spacing of at least one slot between cards and with the right case I can move air efficiently through and out. If they are too close it messes up the aerodynamics completely. It's also helps to use a case with 8+ slots for that extra space.
When the cards are sandwiched with no space the trick is to use blower style cards, which eject the hot air directly out the back. You'll notice though on my 4 card setup the last card is actually a 7970 triple fan card, which works because it has room to breathe between it and the psu.
Ofcourse undervolting, underpowering and underclocking helps control temps too, while still getting great speeds (to my surprise). My best rig runs 3 7950's at under 70 degrees each, generating 1.55mh+ litecoin mining at 525 watts stable, about 3KH / watt. And if I throw in a couple risers I can probably get it near 4KH / watt.
That being said an open air setup with risers will likely be cooler and can be maxed out, but I find that if you are paying electricity and not looking to over speculate and risk frying your hardware, then there is a butter zone which yields best hash/watt, cool temps and yields 80-90% of maxed out settings.
