I would recommend a Nintendo DS Lite with an M3DS Simply
adapter. That's what I use, and it works great. Use Moonshell for media playback (including Vorbis and FLAC), DSOrganize for a nice organizer, and there are tons of great commercial and homebrew games
. Someone has ported Doom (check out http://www.dev-scene.com/
for a good starting place for homebrew info). There is also DSLinux
EDIT: It looks like there's an M3DS Real now with a bit more functionality than the Simply (wasn't out when I got my Simply). Depending on what you want to do, it may or may not be worth the premium (probably $20-30 or so).