ATI will introduce Overdrive 4 with the next Catalyst Driver scheduled in March.

Previously you could only overclock your GPU core with Overdrive, and only by a rather small amount.

Now you can overclock both core and memory and the frequency range will be increased to 75% of the next higher class product.

For example the X800 Non-Pro (400 Mhz / 350 Mhz) could be overclocked up to 456 Mhz Core (400 + (475-400) * 0.75). The memory overclock would be 425 Mhz (350 + (450-350) * 0.75).

This rule does not apply to the XT and XT PE versions of the card and it does not apply across video card generations.

More information at TechPowerUp.