but will 2 off them fit in my case?
What kind of performance are you looking to get out of the build?
You would be much better off getting a high end single GPU instead of using crossfire. The only reason to crossfire would be if you already had a gpu and want better performance, or if you already have the highest end cards.
You are always better to get a single card setup as opposed to a dual card setup, assuming there is a single card that will match your demands, which, in your case there is.
Given your budget, I would recommend a gtx 570, radeon 7850, or a radeon 7870. You could also wait until nvidia launches their new 660 ti.