The Macbook Pro 17" specs state:
Simultaneously supports full native resolution on the built-in display and up to 2560 by 1600 pixels on an external display, both at millions of colors
That means that you can have 2560x1600 pixels on the external monitor, for a total of 4096000 pixels.
I wonder if you could do 3840x1024... It would mean less pixels, but a different layout.
I ask this because this is very very tempting.
Update: sucks… According to Matrox compatibility chart, the ATI Mobility Radeon X1600 is not supported.