For smaller builds (4 cards and under) I will go with Gigabyte GA-970A-D3 ($90)
Either build will get a Sempron 145 CPU ($45) and some cheap 4 Gig ram stick ($50)
GPU selection will be dictated by whatever good deals I can find. Preferred GPU is R9 290 (non x).
PSU selection is another thing since there are so many options and configurations. Some top brands talked about here.
XFX 750W is a well priced one at $90
Calculate PSU needed by using this online calculator. It was pretty close when I calculated my 4 GPU (r9 280x) mining rig running at full load. I was drawing from the wall 1030W and the calculator came up with 943*1.1 (1.1 is for 80Gold PSU) = 1037W.
According to the PSU calc online, a R9 280x sucks 218W, a R9 290 draws 230W and a R9 290x draws 280W.
The CPU (Sempron 145) and MoBo draw 68W at 50% load (lowest CPU utilization available in the calc). In reality it’s probably a little lower since the CPU is barely used.