Its not really the watts that counts, its the quality. If you had a good 450W PSU, you would be able to run a 550Ti no problem.
Good brands are Corsair, Seasonic, Antec, and PC Power & Cooling.
That being said, I would recommend the 7770 over the 550Ti. The 550Ti is quite old, uses alot of power, and the 7770 is just flat out faster.