A big-dollar rig, huh?
A 3770K - Tom's questioned why anyone would need more than a 3570K to do anything at all!
So it's a great CPU of course, probably more than anyone needs (but I like i7 myself).
And 2 x 7970!
I would never advise anyone to put a lot of money into Radeons.
Two nVidia cards will give much better results...
Anyway: motherboards used to support either CrossFire or SLI.
Now, most mobos will support both so this isn't a major consideration.
I personally would select a top-quality mobo from either Asus or Gigabyte; anything with the Z77 chipset.
Probably full-ATX, though many Micro-ATX mobos can run 2 vidcards these days (if you're into small).
Gigabyte GA-Z77X-UD3H is good, at around $145 bucks.
Or, Asus has all kinds or different P8Z77 mobos - I just used one for a 3570K Mini-ITX build. Sweet.
I use Gigabyte for myself - but usually Asus for other people!
Asus is by far the friendliest, bar none...
When you check for a BIOS update they never ask you "what version of the motherboard do you have", LoL, like Gigabyte does.
But (if you're handy) I believe Gigabyte ultimately is the best.