amd recommend minimum of 500watts so if you overclock either cpu or gpu your system will turn off randomly when under stress the psu will be at peak usage and you're risking frying everything ur psu is connected to
upgrade your psu to 600-650 for safety
No upgrade needed. Your system with everything overclocked will not draw more than 375W. You have a good PSU so you don't need to be concerned with overload like you would a low quality PSU. Keep what you have until you're ready to go to X-Fire, you won't hurt anything power wise by overclocking your system as is.
Hope it helps
Edit: Note that AMD does not specify whether the system is overclocked or not - doesn't matter as far as their recommendation