If you are using the Stock Email app, I can agree it is not put together good at all, I know most people who count on an email server other than Gmail use K9 email client it is in the Market, search K9
As far as your screen freezing, this happens to the best of us every now and then, pay attention and see if there is a pattern to it, maybe it is only when your phone rings, or when your phone rings while you were browsing the web...that might help to narrow down the possibilities
The sms notifications might be a setting within handcent (there are so many) did you have this problem before installing Handcent or maybe it came after a recent Handcent update...best way to find out is to uninstall it and text with a friend using the stock messaging app and see if the problem persists, if not than you know it is a Handcent issue
Sometimes an uninstall/reinstall of an app works out any bugs you might be experiencing...reinstall Handcent and text with a friend again if your problem continues Post Back with the details