I've managed to hit 780Mhz stable, but due cooling limit I couldn't get any higher.
As for RBE I really cant remember what was the catch with BIOS editing, I assume there was something, but I had that card back in early 2009 so I do not remember.
Try cleaning with Driver Sweeper and reinstalling drivers after BIOS update.
Here's my modded F2 BIOS for Gigabyte HD4850 512MB GDDR3 card, GPU is @ 700MHz, 1.158V 3D mode.
It worked for me,...