i would suggest you to get the cheapest mobo possible. What you need for mining is just the GPU. nothing else. If you could get hold of a ati 6990. Its awesome, it goes up to 850khash/s at 350 watts. A gigabyte r9 280x goes to 730khash/s. You could buy many r9 280x. and buy cheaper rams/mobos, dirt cheap hard drive of 80gb or something which you can install the windows on. Do not spend too much money on things you do not need when you're gonna use it just for mining purposes. only thing needed for mining is a good PSU, GPU and heat solution
I have 2 miners with these systems:
Core 2 Duo E7400.
3GB Ram
windows 7 and windows 8.
R9 280x and asus 7950
550w psus.
80gb of hdds.
Run without a problem