Originally Posted by ivanlabrie
Easy, z77 gd65, z87 g45, z87x-oc (no risers, yay), z87 oc formula (same), z87 pro, z77x-ud3h, 990fx gd65 maybe.
You'd need water cooling and bracket modding to run 5-6 cards on any board without risers though.
I personally got the ASRock H81 Pro BTC. It's cheap and has all the slots needed as long as you're using risers, but price and availability depends on where you live, as usual.