Well, you can easily watch videos on the n810. I've done it already, although as slha89 pointed out, it's not designed for it. For one thing, it eats battery like it's going out of style. So instead of 6 hours of battery, you're lucky to get 3. Also, you need to convert all your files down to 480x320, 768k bitrate @ 25 frames, 44100 sample rate with mp3 audio at 128k.

If you use "Any Video Converter" (it's a free program) you can batch convert nearly any video format down to the right format. It's even got an Nokia Nseries option on it, but I've found that the custom avi works better as it gets the proper balance of quality over performance. The Nseries option goes with slightly lighter settings for a minor performance improvement, with a slight quality hit.

It's only really a good idea to use the N810 for watching video if that's the only thing you're taking with you. If you can bring a PMP, use that instead for watching video.