Either a larger Mid-Tower ATX or a full tower ATX.
That being said, please don't buy a 680... it's only 5% better than a 670, for $100 more. When overclocked, the cards come out virtually the same in everything but calculations for things such as folding@home.
You also shouldn't buy a TOP edition anymore, if you can even find them. When everyone is still figuring out how to make the cards, Asus pulls the best rated off the line and sells them as the TOP editions.
Manufacturing has gotten to the point, however, that a card produced recently will be of better standards than an older TOP version.