sony vaio and projector

[Amadeus W. M.]

I'm trying to get my sony vaio laptop to display correctly on a
projector.

The laptop has a wide screen and normally X runs at 1200x800 while the 
projector has the usual 4:3 aspect ratio. I read the projector could do
1024x768 but I couldn't get it to do anything finer than 640x480. 
On the laptop I'm running compiz, with the nvidia driver. 

I booted the laptop with the projector plugged in and I did get X to
display on the lcd, with no changes to the xorg.conf. Due to the
difference in resolution between the laptop and the lcd, the xclock
appeared about 1/4 screen on the projector, and a shell took up the whole
screen (on the wall). I added

Modes "640x480" 

in my xorg.conf to lower my laptop's resolution - didn't help. 

How do I set this up correctly? Do I have to configure dual head monitor?
Does it depend on the projector? On the fact that the laptop is a sony? 
I have a major talk in less than 2 weeks and I have to give some live
demos, so I'm in dire need for help. 

Thanks!