matching up laptop GPU modes and monitor screen res

I'm considering a 20" external monitor for my Asus F3Jm (7600 GPU, DVI-D).
The laptop display has a 1280 x 800 screen... the monitor has a native resolution of 1680 x 1050. The GPU's allowed "modes" are 1600 x 1024, 1900 x 1200, 1280 x 800, etc., ...but no 1680 x 1050. What will happen if I try to hook up this external monitor with the GPU at 1600 x1024?
I'm trying to run the external monitor at native resolution for best image.
I would guess that a 1600 x 1024 image will display on the screen with a 40 pixel black bar on the top and bottom and a 13 pixel black column on each side, but only if I specify that the image would display as "centered"
(i.e., not scaled in any way).
Would it make more sense to get a 1200 x 1900 display (i.e., 23" or 24") to fully utilize the available GPU "modes"?

Thanks...

 

thats strange... all the numbers doesn't looks right.... the standard 16:10 are not supposed to be like that. that 1600x1024 should be 1680x1050 and the 1900x1200 should be 1920x1200.

 

Oops - I should have typed 1920 x 1200.
After searching a bit, I did find a "custom resolutions" window that allowed me to define a 1680 x 1050 resolution.
But, for some strange reason, 1600 x 1024 was in the standard list, along with a few more oddball resolutions that don't match up to any 9:16, 10:16, or 3:4 aspect ratios.

 

When you have nothing plugged into the vga/dvi port, then the resolutions available are just those.

But at least for me, when I plug in the LCD and enable it, all of a sudden I have A LOT more res options available, including the 1680*1050 that my monitor takes that wasn't an option when I didn't have it connected.

 

Aha. That would explain the "default monitor" on my as-yet-unused monitor 2. And the odd default list.

Thanks.