Crossfire HD 7850 vs R9 270x

[SantaOnRs]

I am buying parts for my first gaming PC and I want to know if I should get Crossfire HD 7850 or a single R9 270x.
As I am new to building PC's here is my parts list because I'm not sure if it'll make a difference to what card/cards I should buy.
http://uk.pcpartpicker.com/p/VhTLwP

 

[Gameduck]

I'd recommend buying the R9 270X, i prefer a single high-end card over two mid/low-end cards and the crossfire performance isn't that good!