I tend to agree with those who say chop it back a bit, if the limits are set at 200 or 300, you may be heading for trouble when the forum gets big (and im sure its going to get very big!)
How about a little user training, not sure if people are aware but with Vbulletin, you can scroll to the bottom of your PM page and download ALL your pm's and you can choose the format.
Why not let people store their own messages and take the pressure off the server, it has enough on delivering the things, let alone looking after a mamouth database because of generous storage limits.
Just a thought. why dont people try it though then offer feedback as to how it works.