Would there be any noticeable benefit in going for the Gold 750 over say the Platinum 660?
Or gold 650 should be more than sufficient.
So a 670 maximum power = 170W
A second generation i5 or I7 OC about 140W
Other peripherals, etc. About 70W
= 550W with everything going full blast
If you are overclocking both 670 then that is maybe another 60W.
Realistically you will never be able to come near pushing everything at maximum load simultaneously.
So get something that comes near or above 650W on the 12V rail.
Make sure it has at least 4 PCI-E connectors