I was unsure if you wanted to stick with your GPU budget so i went for the card that gives you outstanding performance for your dollar. And this is a better card.The 270x is a rebadge of the 7870. This is the 7870 XT.Much better card for the money.
The case is subjective.But this is a good case to start with.ATX build
EDIT:Changed the PSU and OS.Chose Win 7 the first time round