I have a sapphire 5850 toxic and 5850 extreme, I flashed the toxic back in the summer with the 5870 bios to give it 1.163 vcore, 850mhz core and 1200mhz mem.
I wanted to have the same settings on the extreme, but trixx 4.0.1 doesn't play nice in crossfire, and just caused my PC to freeze/bsod whenever I tried changing the voltage, even just 0.01v (even after trying many combinations of settings such as no ULPS, no syncing of video card settings etc.).
If I take out my toxic, trixx works fine.
After having a play around with the voltages, I decided to try flash the extreme to the same settings as my toxic with RBE+winflash. I dumped the extreme's bios with trixx, and tried changing the 0x46 register to 1.163v AND then used the overclocking function to get the core/mem to 850/1200 on that voltage (I then saved the bios, and flashed separately with winflash).
This half worked, it made the voltage go to 1.163v, but the clocks were stuck at 400/900, while idle or under load. I could change the frequency in trixx, but not afterburner. lame.
The way I have it set up now: changed ONLY the 0x46 register to 1.163v and left everything else alone. Now just overclock with the CCC overdrive from 725/1000 to 850/1200 and use afterburner only for monitoring.
Both now idle at 157/300, and jump to 850/1200 under load.
3Dmark11 score: P8072
Hope that helps!
EDIT: afterburner works again, yay! 900/1200: P8400