Currently I have just 1 radeon HD 7950.
I am planning on using 2x radeon HD 7950 with a dedicated physX card (like 650 ti) sometime in the future. So I am looking for a Motherboard that has the capability to do so (future proofing my system for physX heavy games like Blade and Soul and other possible titles).
Crossfiring might be an overkill tho.
Most motherboards will say specifically if they support three way SLI (not all though). However, You cannot SLI/crossfire a radeon card with a nvidia one - they're different brands. So, you cannot put a 7950 (or two) with a 650Ti; it won't even have the right connections. But two 7950's should be pretty amazing power.
May want to look, Asus have some good choices in the z77 chipset, or Asrock. you may want to make sure you get USB 3.0 support.