Replaced my HD4850 with a HD7770. The old card worked perfectly in ML and Mavericks, except for the occasional ID modding.
With the new card, the first problem of booting from a GUID disk was overcome with a boot0af file. The second problem was when HWMonitor was launched, system crashes and was overcome by yanking out GPUsensors. The third happened when waking up from sleep, an empty desktop bereft of icons appears and system freezes. This can be resolved by replacing the AMD kexts from ML but the white screen problem reappears as in ML.
Anyone out there having similar Wake from Sleep display freeze and system crash with HD7XXX cards in general or HD7770 in particular and is resolved.