Back to the original question...
I have five rigs, 4 with 4 GPUs, one with 3. A degree of mix & match between 7950s and 7970s to keep within power constraints (and I've had a mains cable melt

) I have a few 7990s for another project.
A rig is an open-plan unit with a motherboard, an open frame to hang the GPUs, and one or two power supplies. Milkcrates are used extensively out there. Use good-quality PSUs; you will need powered risers to avoid burning the mobo, and plan on around 2.3Mhash/s per rig (your mileage may vary). I use Win7 mostly - Win 8 may allow more GPUs but it sucks. Team Viewer is a godsend. CGMiner and CGRemote are both free and very useful - one system is automatically turned off during daytime hours using CGRemote. Fine tuning for optimum hashing is black magic. Difficulty is going up exponentially as everyone has a go, and GPUs are getting hard to find - I think AMD will have an appreciable boost on the back of Scrypt crypto mining.
Before you even start, answer three questions for yourself - Space, Heat, Power. Where are you going to put them - avoid dust, water, near a good power source et cetera; Heat - how will you manage what is a effectively a 1.5kW fan heater on all day & night; Power - how are you connecting it, and what will you pay.
If you live in a cold part of the world your rigs are ideal for home heating. Up here in the tropics - not so much.
An ideal situation is to have a honking solar with spare capacity that you are already writing off as an asset.
The theoretical ideal card is an R9 290X - good luck finding those, and the reference design airflow sucks (single fan. Ho Ho). If you can get hold of 'em (and I suspect the US has already been sucked dry and Oz is pricing 'em high) you would probably find that they would spend a lot of time throttled down.