When you get a power supply you need to look for 2 things, is it CE, UL, etc rated , and is the rating on the 5V and 3.x V lines ok for the system you are building. So,e supplies are rated 600W but only on the 12V line used for HDs etc and not so much on the lower voltages used for the CPU and video card. So a 650W supply might only be as good as a 450W supply from somebody else.

The rated supplies also tend to shutdown or not start when shorted out, and are tested at the rated specs. There are cheaper supplies that are not rated by nay agency and those are suspect.

Unless you are using some power hungry CPUs along with multiple HDs and SLI video setups its not a big deal picking out a PS.