not a problem at all. The 550 ti requires 22amps on the 12v rail, so you should be good for now. When you look at upgrading further, you should analyze the needs of your system with http://extreme.outervision.com/PSUEngine.
you can also use that tool to check your system now, if you like. Just put in the parts you have in your machine and click calculate and it tells you the total wattage your system should draw and gives you a recommended psu wattage. I think you will definitely be fine on your current psu until you start to upgrade more.