crossfire with that setup is a horrible idea. two reasons.
1 microstutter. if your not familiar with it look it up. it makes games look bad.
2. virtu mvp. your board supports it. again look it up if your not familiar with it. it makes games run so smooth its a wonder why we didnt have it before. of course you cant set your game to ultra with a crap card and get anything playable but its very nice for proper settings.
so i think you should have a single card. a 7870 is twice the price of a 7770 and twice the performance. but uses less power, can be crossfired later on if you feel the need. and if you get one with reviews showing its quiet you will have happy gaming times.