I don't usually upgrade unless there is a feature in the new OS that I want. I have a Bold and I'm at lest a version behind the latest. Although upgrading is easy, probably best to use the BB first. To find your version, go to options, about.
Start here: BlackBerry FAQ - BlackBerryFAQ
Check out the 8900 subforum on this board too.
Preserving battery life, there's plenty of information on this board so do a search. Software that uses GPS (Google Maps), IM software, Twitter software can all take a toll on the battery.
Unlimited data plan (on AT&T anyway), you can pretty much email, browse the net and run third party software as you want. What you don't get is SMS/MMS as this is a separate messaging package.
Good luck, it is a nice phone.