I would recommend 8 with 6 GPUs. also don't use a total shit CPU. Use a intel sandy bridge series processor at the least. doesn't have to be a good one, but i'd recommend it. With the recommended boards you've seen such as the MSI Z77A-G45, a sandy bridge processor is a must. Also, I recommend using a server power supply (1600W min). Or just ground two 1000 W PSUs together.
Honest question, why do you need a Sandy Bridge?
I was under the impression that he was going to do some Scrypt mining. Sandy Bridge processors aren't crap for it from what I've heard.