If you install wireless-tools (apt-get install wireless-tools) on your N800 you can see the details of any surrounding wireless networks (including channel numbers) by running (as root) the command "iwlist wlan0 scan".

When choosing a wireless channel, it's best to choose a channel with a seperation of 3 from any other channel (ie. choose channels 1, 3, 6, 9, 11) - channel 2 will continue to receive interference from channels 1 and 3 etc.