Ok I figured it out. I had to get the ATI control panel or whatever. At first I downloaded the Windows XP version, which is what I thought I needed because I'm using Windows XP. It wouldn't install, so upon closer inspection of the ATI website, it turns out I had to install the Windows Bootcamp drivers. From there I switched the Samsung to the primary monitor and disabled the monitor on my MacBook Pro. Even better, it switched seamlessly back to the MacBook Pro monitor when the Samsung is unplugged, and when the Samsung it plugged back in, it goes back to my custom settings. Yayness!