Thanks for the ideas everyone.
I'm going to take the bike in to the bike shop before RAGBRAI this summer, so I'll try on some different helmets for fit there. Considering that there is a lot of variation in fit from one kind to another, I don't think I can just order one form Performance Bike or something. About that impact question, I really don't think that the helmet took any major blows. The shell is just a little chipped in a few places, but it's enough to bug me. Also, I think that newer helmets have more and bigger vents, and are probably cooler as a result. :)