Pretty much this. That case is tiny, and beyond that the cards you've chosen don't expel the hot air out of the case, but just circulate inside the case. Seeing as how you haven't built this, and just compiled the parts I can see how it would be easily overlooked. But I can guarantee that you will have serious overheating problems with those cards. If anything I'd switch to reference designed cards, so at the very least a majority of the hot air will be pushed out by the card fans. Even then I wouldn't expect to be able to overclock much and maintain safe temps.