Try this tweaker to lower your memory and core timing dude. (Overclocking to the limit will cause bad screen tearing on radeons).
http://hem.passagen.se/emiper/Radeon.html

165/165 sync clocks is standard for OEM.
185/185 sync clocks I think is standard for Retail boards.

You can pretty much tweak the lot with this little baby though.

A excellent tool to fry a Radeon with... LOL