Nvidia recommends a 400w power supply for a system with a GTX 550 Ti. That assumes a generic "junk" brand. So with a quality unit anything 350 watts and up is probably fine. Stick with Corsair, Seasonic, PC Power and Cooling, XFX, Silverstone, Enermax, OCZ and Antec.
The GTX 550ti doesn't take much, it draws about 12.67A @ +12V at max. That's about 200W max. Something you will rarely ever do. The CPU maybe 5A. So look for a PSU with the total wattage needed by your system using this calculator, and be sure it has a sufficient +12V rail(s) to carry the load, which most brand name PSUs do. http://www.thermaltake.outervision.com/