You will need to edit the default mode listed in your Xorg /etc/xorg.conf
It's not hard to find a how to on xorg.conf
My setups native mode is 1024x600 and with the external monitor attached you can configure much much higher resolutions with he external adapter by modifying the external display script that comes with the eeepc-acpi-utilities
Edit: maximum external res for the Intel GMA 950 set is 2048x1536 @ 75Hz.... not too shabby



