I had the same question when I was ordering a 2005FPW to go along with my 17" Powerbook. As long as your graphics card can drive the native resolution on the display, you will be fine. For my 17", I see the following behavior when I plug the monitor into my DVI port (I've only tried DVI, not the standard VGA connector):
Mirrored mode: I see the 1440 x 900 output of my Powerbook mirrored onto the 2005FPW. This means that since the native resolution of the 2005FPW is 1680 x 1050, I see a black border around the mirrored 1440 x 900 output.
Spanning mode: I can independently set the resolution of my 2005FPW in its spanned configuration. So while my Powerbook LCD is at 1440 x 900, I can drive the 2005FPW at its native 1680 x 1050 -- it looks great.
Hope that helps!