Lmao, temperatures with 2 x 390x in an mATX case will be horrific. Plus, XFire 390x will perform on par with a single GTX 1080, at best, with really good scaling. Then you have to take in to account that plenty of games have poor, or no multi-GPU scaling, and the fact that you will undoubtedly have more stuttering/worse frame times with a multi-GPU set-up. A single GTX 1080 is hands down the better option unless your mining cryptocurrencies.