1600x1050 Fullscreen Resolution?

I simply cannot get this to work. I've edited the xorg.conf file, I've even tried using these boxed versions, and I simply can't get this working. I've added the screen resolutions to the VM as well, but nothing but the standard 3 resolutions ever show up in the drop-down. Running this on a MacPro with a 20" Samsung widescreen monitor.