And get a decent PSU from the start, the XFX you've choosen is neither very powerful nor priceworthy. For a few bucks more you get something like a Cooler Master G750M, Corsair Builder Series CX750, LEPA MaxBron 800W or even a XFX P1-850S. In other words don't put money into something with less than 750W and 60A on the 12V-rail if you plan to enter the multi GPU path. You can never have too much power and those PSUs should power two 280X and a normal CPU (hence no overclocking, TDP <90W) without a hassle.
And check out the mainboard having two PCI-E 3.0 slots powering at least x8, i.e. the ASRock Z97 Extreme 3. Good for two GPUs. The more GPUs you're planning to install the more you have to plan in advance.
And the cooling of the GPU is essential, if it gets too hot it'll start throttling. So if you don't want an OEM design check out some tests and reviews and buy the card which runs coolest. Stuff enough fans in your case and install something like a power switch to fire them up to full speed when doing hashcat cracking. Also here too much cooling never hurts (except your ears).
And check out the mainboard having two PCI-E 3.0 slots powering at least x8, i.e. the ASRock Z97 Extreme 3. Good for two GPUs. The more GPUs you're planning to install the more you have to plan in advance.
And the cooling of the GPU is essential, if it gets too hot it'll start throttling. So if you don't want an OEM design check out some tests and reviews and buy the card which runs coolest. Stuff enough fans in your case and install something like a power switch to fire them up to full speed when doing hashcat cracking. Also here too much cooling never hurts (except your ears).