Yes the stock Sapphire HD 7850 is unlocked so can be overclocked.......lots of guides on-line on how to do it with software like MSI Afterburner (not sure if Sapphire have there own OC'ing software?)
Sapphire does have their own software called "Sapphire Trixx," but MSI Afterburner is probably still the most popular. I've never used Trixx before, so I can't really comment on it.
They Have TRIXX and its good, for some reason i was having problems with Afterburner and when i switched to TRIXX poof, problesm gone, it also allows you to go over 1100Mhz on the core!