Here is my two cents:
CPU: A4-5300
http://www.newegg.com/Product/Product.aspx?Item=N82E16819113283Why? Cheap cpu, two cores (just in case) and as a bonus, you can get a few extra MH/s with the APU
Motherboard: anything with 3x PCIe x4 or higher
http://www.newegg.com/Product/Product.aspx?Item=N82E16813157339Why? quad rigs are harder to get the parts for and harder to keep cool. They are also harder to get a powerful enough PSU for
PSU: Any 1000W Corsair or Seasonic
http://www.newegg.com/Product/Product.aspx?Item=N82E16817139014Why? Because the PSU is the last thing you want to cheap out on. not enough power, or a bad PSU means an entire rig is down. Another reason to stick to 3x GPU instead of 4x GPU rigs.
Seasonic makes the most reliable Power supplies. Seasonic makes the power supplies for Corsair.
RAM: 4GB of corsair or mushkin
http://www.newegg.com/Product/Product.aspx?Item=N82E16820233349Why? Because it's cheap and I trust mushkin and corsair. I have no real basis for this trust though.
Case: make one yourself out of wood. Set it up in a rack style with 1 system on each level. Set it up as an open-air system.
why? because it is fun and will be easier to manage than what you could buy
storage/OS space: a hard drive. Western Digital blue or black
http://www.newegg.com/Product/Product.aspx?Item=N82E16822136567why? because I have good experiences with WD warranty. They have an option where they send you a new drive first, then you send the defective back. This will reduce your downtime because of a failed drive.
and finally....
The video cards:
You want 3 for each system. Get them where you can and get the best deals you can find. 5970s, 7970s, etc. This is where you will spend the rest of your money so balance it out accordingly. There are no issues with mixing and matching video card generations as crossfire is not a part of mining.
Consider cooling. I personally would never use a reference cooler for mining. Also, XFX single fan coolers are total garbage. My experiences with sapphire custom cooling have been good. I also read a lot about and if I were to purchase a 7970, it would be one of the 3-fan gigabyte ones.
I would recommend installing and running linux to get the most out of your overclocks.