Some 4850s have a 3D volt of 1.16 or 1.18, You can get upto 1.2V if you edit your GPU BIOS. If you need more voltage you will have to do a hardware mod(like Pencil mod or soldering mod)
Just remember you can kill your card if you do the hardware mods. And If you mod the BIOS you can turn it into a doorstop.
Disclaimer From W1zzard at techPowerup! on the HD4850 Voly mod pages
Performing these voltmods will void any and all warranties your video card may have.
You - and only you - are responsible for any damage caused indirectly (or directly) by these modifications.
From Me:
I am not making you do the mods and I will not be responsible if you
damage your hardware.
Without any volt mods, 675/1100 is pretty much an average max. Even after upping the volts on my 4850, I really couldn't get 700MHz 100% stable (run Furmark to see if it's 100% stable) So I back the volts down to 1.15 and settled for 685MHz/1100 memory. Good luck!