I agree that 7950 or 7970 is a good start. I suggest you to get two 2 7950 GPU cards plus a AMD APU and 16 Gb RAM.
So you would buy an AMD APU over say AMD Athlon II X2 270 Regor 3.4 GHz 2x1 MB L2 Cache Socket AM3 65W Dual-Core Desktop Processor?
If my rig was just going to use two GPUs like the 7950 or 7970 then which AMD A4 or AMD6 series processor should I get?