The n800 only supports video and audio chat via 3 ways:

1. Audio only from skype to skype and of course any phone if you have skype out

2. Audio only from gtalk on n800 to gtalk on pc and audio and video for gtalk n800 to n800

3. Audio and video for both pc and n800 via gizmo.

All other addons like msn, yahoo, aim etc can be added to the built in chat client via rtcomm. This only allows instant messaging.
Another option is a third party client being pidgin which as far as i know only supports IM

Please correct me if i am wrong