The new PSU will do nothing but good. The bad things that could happen are if it is a cheap PSU, if it is a cheap PSU it could damage your computer *cheap PSUs often state that it has 200+ more wats of power then it really has and when your computer trys to take more then it can deliver it frys the system*
A good 550W PSU will easily power your whole setup including Crossfire with another 5770.. If you have money to spend and want to spend it on getting a high quality PSU (always a good thing) then i recommend you to get a quality 750W PSU which will easily take care of your future upgrade plans also..
It is worth the upgrade imo- though as others have said, I would suggest a *good* PSU over a high wattage PSU. There are some very good models that advertise 750W, but can draw much higher without danger.
I had this same question and went ahead with the nicer PSU and got an XFX 750W BlackEdition (Seasonic base)
What isn't widely advertised is that you will gain efficiency by using a PSU at less than its rated load, so all things equal- a good 650-750W PSU (Seasonic X750 or X650, Corsair HX Series, the XFX 750 BE, to name a few top tier) powering a 450W system will generally produce less heat (efficiency loss) than a good/decent 500-550W PSU would on that same setup. Less heat and more efficient is nothing but good news.