I guess I neglected to mention it because using an external monitor has no impact on performance.
In fact, the laptops I looked at even used 1920x1080 for their native resolutions, which seems high to me considering I'm using the same resolution on a 23" monitor and the laptop only has a 15" screen, but I suppose they wouldn't be using it if it didn't look okay.
The only reason using an external display would lower your frame rates would be if it were set to a higher resolution, because it has to render more pixels for every frame, but it would be the same as if you increased the resolution on the internal display.
Here's a link to the laptops on iBuyPower to give you an idea of what to look for and the prices. I wouldn't personally buy a laptop with a 17" screen though, at least not if you're planning on moving it around and why would you buy a laptop if not for portability.
Keep in mind you will need to make sure the laptop you buy has a port for whatever monitor you're plugging in or you'll need to use an adapter (generally hdmi, dvi, or vga). I wouldn't pick the laptop based on this though, I'd get the laptop I want then adapt the monitor to suit my needs.