Since your upgrading away from a 550TI there is no need to worry about you PSU since you won't be using that card anymore. If you are going to upgrade your PSU make sure you buy one that it at least 80 Plus Certified. These models are less prone to losing power when it is drawn from the outlet, converted (which is where lower end PSU's often lose power), and finally reaching your computer.
The reasons for owning an 80+ is because you are often times getting the power that you paid for, not 550 watts minus whatever is lost in heat transfer.
But for most purposes if your only using a single GPU 550 watts will be just fine!