Hello!
That is not a overclocked system at the first place.
2*hd 7850 draw much lesser power than 2*hd 6950.
@yochan
Power requirements for graphics cards
7850 Crossfire - Will it run on a 550w Power Supply?-page 2 anandtech forums
GS600 can deliver up to 48A on a single +12V rail.
hd7850 draws about the same power as hd 6850
According to reviews the hd7850 with an
overclocked i7 2600k draws less than 300Watts under max load.
Even if you add another 200watts(though another hd 7850 won't draw that much more) it is still less than 500watts.
Moreover i5 3450 consumes much lesser power and is not overclockable.
There is enough room for overclocking.
Overall gs600 can handle hd7850 crossfire with your config.