-If you want a fancy motherboard (I'm assuming you are since you listed a 790FX in your theoretical build, and because you want crossfire) - then I recommend going with their newer 890FX boards that came out recently such as the ASRock 890FX: http://www.newegg.com/Product/Product.aspx?Item=N82E168...
[If you don't like ASRock, there are other brands that have 890FX boards as well].
-If you insist on CrossFire, get two 4850 1GB cards in crossfire for $220. This CF setup generally performs at or above a 5850. Those 890 mobos will be great for CF as well. Otherwise the build looks good.
[You could also get 2 4870's for $300, or 2 4890's for $400].