I'll be building a mATX in the next couple weeks. I am using the NZXT Vulcan, just to throw out another case option for ya. Crossfiring shouldn't be a prob with the Vulcan. Although, with the 5850 you shouldn't really need to worry about that.http://www.newegg.com/Product/Product.aspx?Item=N82E16811146065
The MSI mobo is basically about the only option for crossfiring in the mATX category. Hate the lack of options there.