Get Gigabyte D3A mining board or even better Asrock h81 r2.0 mining board, i belive its more plug&play. You might have some problems getting the 6th card in Gigabyte board.
buy 6x amd 570 4gb. You will need atleast 1000w psu. maybe get 2x 700w.
how much mh is the total of these 570 amd 4gb? Also how many psu i need 2 only? Tell me how much power they will use (for electricity).
Also what i need for 300mh?
You need more powerful PSU's if you are using AMD cards, coz nVidia uses less power. For my 8 x 1060's i get 195 MH consuming 700Watt though I have 2 x 750 watt evga gold psu.
I would suggest to get 12 or 13 1060's or equivalent other models of nVidia to get 300MH.
Its always better to get extra watt on PSU which is just like insurance, will cover your PSU's life.
Tell me how much you paid for your rig? I mean how much for gpu,motherboard(which one u used),parts,2x750 watts evga gold ?