If I start with them, can I transfer all the posts to a 'fancy' site?
Someone correct me if I'm wrong, but all the big message boards have converters which are straight forward and easy to use.

Does this one upgrade while keeping the info?
Yep! All the info is stored in a database, not the files, so when you upgrade your usernames and posts stay right where they are. phpBB has pretty good documentation for upgrading. If I can do it, anyone can.

As for the best, I won't go there. But I have used phpBB and never had any issues. Hope that answered you questions!