The Antec 900 & 1200 cases are both nice, but the 900 fits into a $1000 budget a lot easier. The 1200 is quite a bit bigger than the 900, but the 900 would be plenty big for the above build.
I'm guessing by "the thing that allows you to use both cards for gaming? " you mean crossfire? Yes, the motherboard I linked above is crossfire compatible.
A 3GHz OC is pretty easy on the Q6600.
I'll let you pick your own keyboard and mouse.