775 is the max ?
sapphire trixx will let you oc one card past the limits other programs have.
i wasn't able to get afterburner to clock past limits either and wound up using radeon bios editor to flash my cards. ( it's painless ) http://www.techpowerup.com/articles/overclocking/vidcard/154
You'll need to download GPUZ to easily save your current card's bios file. Then open the file w\ RBE and make changes, and save the updated bios file. Then use ATIwinflash ( dl from within rbe ) load the updated bios, select card, and click 'program card' takes afew seconds and it will now be flashed. tell it not to reboot and flash the other card. reboot and you should now have OC'd cards without having to mess w\ programs.
Afterburner is still great for it's GPU usage and temperature displays + fan control