Here's a short list of mATX Z87 boards that support SLI : http://pcpartpicker.com/parts/motherboard/#s=24&l=2,3,4&sort=a6&f=7&c=89
If you prefer better sound quality, pick the Gigabyte G1.Sniper M5. If maximum overclocking ability is what you're heading to, pick the Maximus VI Gene.
Anyway, a pair GTX780 cost just 1300$ and is less than 10% worse than a pair of GTX Titan ( 2000$ ), and if you OC then a pair of 780 can match a pair of Titan in no time, so you should consider lowering the graphic cards to a pair of gtx780. Doing that should save you a lot of cash for maybe a custom water cooling system without sacrificing too much performance.