Judging from the game's release date(which is tomorrow), I would say it will require at least version 3.52 or 3.60. 3.71 just came out recently, and I'm guessing that production of the game began before the latest firmware was released.
Right now I'm on 3.52 M33-4, but if the game requires me to update, then I'll have no choice but to update to 3.71 M33-2, which is still unstable at the time being.
EDIT: @Thomas Belmont: Restoring your PSP back to its factory setting won't change your firmware at all; just your settings. There is a way to downgrade any firmware, however. It's called Pandora's Battery. Not only that it can downgrade any PSP, but it can also unbrick any bricked PSPs as well. Search it on google.