http://www.overclock.net/t/96712/how-to-jump-start-a-power-supply-psu-test-a-power-supply-and-components
This works just as well as the add2psu if youre keeping your pc on all the time.

For 5 7850's I'd just get a good quality (seasonic,corsair,?) 850w psu (SINGLE RAIL). Should be enough unless your cpu/periph. situation draws too much.