go to the Actions window, select New Action. hit the round record button, do your actions, hit the square stop button. Name it whatever you want. You now have a custom Action.
Go to File, Automate, Batch...
The set should be the default set. the action should be whatever you named it. choose your folder, choose your destination folder (it should be different so you won't lose the originals in case something goes wrong)
and hit OK.
It sounds confusing, but it play around with it for a few minutes you'll figure it out. It's truly a wonder of Photoshop 5.5.