the 550ti i have it on a cheap 500w ps and it fine. (gaming). the 550ti is going to pull min power from the ps at post. your going to be running the bios screen that vga mode not windows aero or 3d mode. what you want to look at is the 12v line the wattage and amp rating. you need the amps for the video card and the cpu. (take the wattage and /12v to get the amps.
so if the cpu is 125w...or 12amps. http://www.bit-tech.net/hardware/graphics/2011/03/15/nvidia-geforce-gtx-550-ti-1gb-review/10
says 140w in windows. looking at another 12 amps or so. so check your ps to make sure there 24 amps or more on the 12v rail.