I'm running 1650x1050, and as BlauK said I lose some vertical. I can't see all of the gauges, and I have a very limited high-12 view. I can manage with it, but it's always a trade-off.
I'm still waiting for the 2-monitor support in AH.. I have a 17" 1280x1024 monitor on top of my 20" 1680x1050, and it's really great in X-Plane. I can have the gauges on one monitor and main view on the other. Or have both views as 3D views e.g. one from outside, one from inside.