Overlay Resolution

So I'm using two monitors with a 6600GT and dualview. I like to have the video out fullscreen on the second monitor. Except it displays overlay mode at 640x480.
How can I change it to 800x600 or 1024x768 ?

>>4
Other than the fact that the LiveCD manages to be the slowest piece of crap in the world, that still doesn't solve the problem. In fact I'd rather not spend 8 hours configuring something that would bring the entire system crashing just because I felt the need to use linux.