First GPU Rig - A few questions

[Flomac]

Awesome thanks for the detailed info, I think that was the case I was looking at older forum posts,
the only thing that mislead me was this website: https://blog.netspi.com/gpu-cracking-rebuilding-box/
their build was running 6x R9 290 with an Intel Celeron 2.7Ghz and only 8GB ram.

Karl from NetSPI obviously doesn't know what he's talking about. Ask him for a real world hashcat job instead of posting simple benchmarks. There are many ways to do hash cracking and a 2.7GHz Dual-Core feeding six GPUs is limiting you from most of them.

On top he's dealing with a lot of heat from the six 290x as he writes in his blog, well why not using reference cards as it is claimed here since years? And put at least some fans in the case. Then your benchmark might stop showing lousy 10.4 GH/s in MD5 - almost 10% under the expected value.

So again, that blog is at no point a good advice for building a rig.