Yes, you can use your n800 with slimserver. In fact there is a slimserver skin for the n800 (actually made for the 770), which on the build of slimserver I have, is at: 192.168.x.x:9000/770/

You just point your mediaplayer to stream:

192.168.x.x/stream.mp3

and control it with your browser at the above address, where 192.168.x.x is the IP address of the computer your slimserver software is running from. Your build of slimserver may not have the 770 skin, so just point it towards:

192.168.x.x:9000

If you have more questions feel free to ask. There should be some info on old posts too that you should search.