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.