Ok
MoboGet one with 4x 16xPICe rails. All those 16xPCIe rails must have another rail in between EDIT: or a space at least. CPUs don't compare with GPUs for mining. If it says it can do 4-way CrossFire or SLI you are good to go. :-)
ExamplesIntelGigabyte UD9(GA-X58A-UD9)
Asus Rampage III
Asus P6T7 Supercomputer
EVGA Classified 4-Way SLI
AMDMSI 790FX-GD70
..dunno of any more, can anyone help?
Graphics CardThere is only one option really, the ATI Radeon HD 5970, it gets you around 550 MHashes/sec.(Sorry, can't tell for sure. I got a 5870 and I get around 300MHashes/sec) They cost like 600 dollars each and you could be doing 600 MHashes/sec easily By OCing the GPU and UCing the memory. Which means it will cost you around 1 USD per MHash/sec + electric bill of it running. Also if you plan to get more than 2 in the 4-way mobo then the liquid cooling upgrade is mandatory.
CoolingI suggest you get the Koolance block for the 5970. Refer to this review please:
http://www.youtube.com/watch?v=7HfXEA2EJpIPSUGet the biggest one you can get.(1500 Watts) Corsair has a big reputation for its reliability.
CaseMake sure you have at least 8 slots in the back for the GFX.
CPU and RAMGet an economic one since the GPUs will be doing the job. Same goes for RAM.
Operating SystemI heard that Windows has a limitation of 4 GPUs(You will have a max of 8 per rig) so I suggest you use a Linux distro such as Debian or Arch Linux.
I hope it helps.