i have no idea why people are saying it won't:
1 x 6 pin + 2 PCI-E
and with a 18 amp and 17 amp 12 volt rails there is "supposedly" enough power to support a 7850; that uses ~110 watts or less.
i say supposedly because being an unknown brand the label may be false, it happens with "unknown" PSU manufacturers. IF you are going to get another PSU look for a corsair, antec, XFX or seasonic. a raidmax isn't any better than what you have now.
Your PSU isn't some super cheap Chinese piece of crap and should work as specified.
It will be enough don't listen to this people screeming you need corsair blah blah that's probably only brand they know.
you have around 150 W draw from the board and ~100 W from 7850 GPU.
You always overclock on your own risk but there is safe 50W headroom with your single PCI-E connector.
From my point of view some hard gpu overclocking is verybad for it's lifetime so go easy on it or you may dump it under 2 years.