I just bought a Gateway Dual-Core Widescreeen M685 (loyal Gateway customer...I've had 2 bad experiences w/Dell). It has a 17.1" display. This is my first laptop...previously I had a 17 inch flatscreen w/ my desktop. I have to say that the display on this laptop is just as good as my flatscreen.
You could definitely work two documents side-by-side on the 17 inch. When I first got the laptop out of the box, I thought it looked enormous, but after having used it for 2 weeks, I can't imagine wanting anything smaller. I haven't ported it around yet, but honestly, it's not THAT heavy.
With rebates any everything, it cost me $1500, which I think was a pretty good deal.
I personally would go with the biggest screen you can get. It's worth the extra pound I might be carrying around.