I would recommend a N800 over an N810 if media is a big concern. The N810 only has a Mini-SD card slot and 2 GB of flash (which is mainly full by default with maps), while the N800 has 2 full sized SDHC card slots, which will let you carry a lot of music and video with you.

As an audio device, the N800 isn't too shabby. There are a number of excellent 3rd party audio players with spiffy user interfaces. I recommend checking out Camola and UKMP to start.

You can find a video of UKMP at Youtube here: http://www.youtube.com/watch?v=nFyJy3_uUGw

You can find Canola here:

http://openbossa.indt.org/canola/

The N800/N810 also has a Rhapsody client, so you can download oodles of audio content directly to the device. The N800 also works with upnp media servers, so you can just leave all your media on a PC and stream it to the N800 over your network. I also use ORB to stream media to my N800 while out and about in the world. The N800 also plays internet radio and has an FM tuner built in.

Video format and resolution support with the default media player is somewhat limited, but Mplayer is also available and is capable of playing a wide variety of formats. Typically, PC downloaded video will need to be transcoded to something ~400x240 to play well in Mplayer. Thankfully, there is a drag and drop simple app put together by Kontorri to make the conversion easy:

https://garage.maemo.org/projects/mediaconverter/

My N800 has kept me happily ensconsed in media on many a plane and car ride. My only wish is for official A2DP bluetooth headset support and the ability to play back high resolution video without transcoding.