External monitor not going full resolution on extended desktop

Hello,

I have a Gateway widescreen 1680x1050 resolution monitor plugged into my 13" Macbook Aluminum computer. When I have both my external and built in display turned on extended mode, my external monitor only goes up to 1600x1000 resolution, which is obviously lower than full-res. When I close the lid of my Macbook (and run the external display only), the external display goes up to full 1680x1050 resolution. Could someone please help me solve the problem of my Gateway external display going to less than full res on extended display mode.

Thanks a lot!
Adrian