Just remember to have a beefy enough CPU with atleast 2 cores to supply work for those GPUs, specially if you go 2x7990 or more. And as I said earlier, 4GB of RAM is not enough for that amount of GPUs on anything else than SHA256 mining. Plan on having +1.5GB/GPU of RAM (7990 might need +3GB/GPU) for scrypt. You can find more info in scrypt mining in other topics regarding CPU and RAM requirements.

Corsair 1200AX is something to consider, someone said not to cheap on a PSU and still they offer 1000W unit when you talk about top-end 7990s and 7970s 

I'm using my rig also for hashcat so I understand your point of not making just a bitcoin miner. I used to run BOINC too some time ago.