Forget software, what you need to do is list each and every bit of hardware that you will now or in the near future be connecting to a power supply. Your UPS that feeds the system will need to match or exceed that. Most of the guys here are able to help you work out what your requirements will be.
But if you are running a graphic card of any kind and possibly overclocking your CPU then most certainly 550W will fall over even if it was working at optimal performance which very few UPS ever do.
edit: The guru of power has spoken, ignore me and listen to what DC has said.
To help us help you - please provide as much information about your system and the problem as possible.