With that power supply, I would go with a 5670. This card is known as the best (or one of) to be able to run on a 300W power supply. A 550Ti will most likely not run on a 300W PSU of any sort. The 5670 will allow you to play the latest titles (save for maybe BF3 and Skyrim) on medium graphics.
Besides, the 550Ti is not really the best in class in that case, I would look into a 5770 or something.