Uh, make sure you have the latest drivers first, by getting your exact specs and downloading the latest CCC package from the AMD website.
Then, once they have installed and you have restarted your computer, there will be a new symbol down in the right of the taskbar, called either "Catalyst Control Center" or "AMD Vision Engine Control Center". Go in there, and find the 'Gaming > 3D Application Settings', then customize to your liking.
It's possibly the easiest control for this, and is the same with nVidia. I thought this was basic knowledge that before you game, you sync up your settings?
There's no way to do it with specific games, unless you tick "Use Application Settings" or similar, and force it in the games settings panel.