This is a little report on the experience I made with overclocking my AMD Sapphire HD 5850 Toxic Edition. One problem is that this graphics card has a lock which prevents you from overclocking (actually you can oc a bit but not much) via software (I prefer MSI Afterburner).
So the first thing you have to do is to flash the GPU with a BIOS that has no such lock.
Warning: This can damage your GPU and render it useless.
You will need a way to boot into a DOS. I’ve already posted a tutorial on how to set up a bootable pen drive with FreeDOS. You can find it here.
To flash you GPU you need a tool called ATI Flash:
Put the atiflash.exe onto your pen drive.
As already mentioned before, you’ll also need an appropriate BIOS. This Asus BIOS worked well for me:
Choose a shorter name for the file (you’ll have to type it in later) and then also put it onto the pen drive.
The last thing you need on your pen drive is a backup of your current BIOS. You can create one with GPU-Z.
Now with all three files on your pen drive, reboot your PC and set the BIOS to boot from your pen drive.
One thing that will help you a lot in case of a failure is to note down every step you have to make to flash the BIOS so you can do it blindly if you graphics card won’t work after the flashing. This way you can reflash you backup easily.
So once you’ve booted into freeDOS, type
atiflash -f -p 0 thenameyouchose.bin.
This will flash your graphics card with the selected BIOS.
Now reboot and if your graphics card still works you can start overclocking. You will notice that you can set all the values in MSI Afterburner freely now.