Here you got everything about flashing/updating:
http://wiki.maemo.org/Updating_the_tablet_firmware

After that, IMO You should install tool called backupmenu, kernel-power v48, Community Seamless Software Update, Enhanced Busybox shell.

Just search wiki (section: community, from top panel) or forum for threads about those - it will also get You used to Community board

If it helped You, no need to post reply with "thanks" - just hit "Thanks" button in right down corner of my post.

And, have a nice time with Your N900! It's really one device of it's kind, and You've probably did one of best investments in Your life