Just to give you an idea how 1080p looks on the HP 2140 with 1.85 GHz:
http://www.twains.ch/Dolphins_1080P.mpg
It's the dolphins 1080p video from the Microsoft HD swowcase. It's 1080p but anamorphic (only 1440 pix wide). It stutters once or twice very shortly over the whole length of the movie. With higher clock frequency and/or a better codec I see really no problems to achieve a seamless playback. Every kind of 720p works pefectly anyway.
Twain