How to switch between two monitors (main and second display)?

I use a Mac Pro (early 2009) (with Lion) with an ATI Radeon HD 4870 graphic card.

On the DVI output I've connected an Apple 30" Cinema display, on the Mini-Display Port I use the Apple Mini-Display-Port Adapter to DVI, and connected via a DVI-cable a Samsung SyncMaster SA450B to it. All works fine, but the main screen is on the Samsung display.

How can I switch the monitors, so that my Apple Cinema display is the main monitor, and the Samsung only the second one?