Open a Terminal Window and type the following: defaults write -g com.apple.mouse.scaling 5.0 "5.0" being the number designating the speed. Change that number to whatever you like. I've got my Magic Mouse set to 150 or so. This does not change the acceleration, just the scaling. Altering the setting via the preferences pane overrides the Terminal command. Log out and back in to activate. For a more complete solution, get USB Overdrive.