My experience with OS X upgrades has been painless, I have upgraded Panther over Jaguar. So if you are not having any problems, most can rest assured that a basic upgrade will get them up and running in the shortest amount of time.
I second the backup strategy but I have to wonder about partitioning, even now that iPartition is out which makes undoing partitions easy, it is time consuming to un-partition an 80 gig or larger drive, and partitioning a smaller drive is not a good idea. Sooner or later after iphoto and itunes libraries grow large, you will want to reclaim tha space, which is why the dual boot seems to be not a good idea to me.
I also want to question the wisdom of advocating splitting up large entourage or Mail databases, or iphoto libraries. While I know that speed is an issue, with the advent of spotlight there seems to be a return on investment for keeping large email databases, simply because the data will be more accessible with spotlight. I am praying for an entourage plugin...othewise I will have to move to Mail. I think the idea of spotlight is to create value in all that old material, and if I can get a spotlight search to return me the entire set of emails to a specific client over the past few years, that kind of information is very useful. While Entourage Archive X is useful, it seems to me to be the kind of app that spotlight makes irrelevant.