For GPU mining a quad 7970 setup gives you 2.6 GH/s.
Right now im running dual 6990's and getting 1.2gh/s.
both of these GPU setups will cost about $1600 just for the GPU's
the 6990's you can run on lower grade MB's and CPU but they cant touch the Quad 7970's
the quad 7970 so far what i have found require a $260cpu and $260+ MB but the MB would be running a x16,x8,x8,x8 on the four PCIe 3.0
If you want to water cool the quad 7970's the whole rig will cost about $3200 the dual 6990's just stock cooling cost about $2300-2500 and you could water cool that rig and get up to 1.7GH/s but that would cost about $2900-3000 all depending on the parts you want.
My 6990 setup is:
AMD FX-8120
Asus M5A97 r2.0
Gigabyte ATI 6990 (x2)
Wifi card
128GB SSD
Coolermaster 1200w 80gold
Diskdrive (for driver CD's)
Corsair 600T
I also added 4 high pressure fins on the side panel