Increasing the frame buffer size

[Dirk Hohndel]

I have a laptop with an Intel GM45.
When I start X without an external monitor attached, I get a
framebuffer size (or maximum screen size) of 1440x1440 as the internal
display is 1440x900. So when I try to connect my 24" external display,
I can't run it at 1920x1200 - that's not so good.

Of course, if I start X WITH the external monitor attached, all is fine.

So the question is "can I increase the size of the framebuffer without
restarting X?"  Or do I have to somehow force a larger virtual display
to have enough space prereserved (I tried that in the config file, but
a lot of things have changed since I was fluent writing config files,
it seems... iow I didn't get it to work...)

Thanks

/D