I'd recommend that you first take some time to read up on Google Voice on its homepage as it will explain things that you ask bout here - like callback number; using SIP, SMS, etc. You say the message layout is 'confusing'. Could you elaborate how its' confusing as just saying it is isn't exactly useful.
SMS works but not short code SMS. So you can't use the 5 letter/number SMS codes (though Google Calendar and Weather work..). SMS sent through GV will use the GV #. On the n800 it alerted you by flashing the blue led indicator. Now on the n900 I believe it ties into the notification system (so whatever LED that uses).