If you will be buying new PSU anyway, then go for 580. Even though you could go for 450 W unit with 1060, price difference between 450 and 550 are often so small, or even sometimes you can get 550 W cheaper. Plus, it is always nice to have more powerful unit so you don't have to buy new every time you swap GPU, as lifespan of quality PSU is much longer then lifespan of GPU.