It will draw whatever is required. You have to have lots of components to draw 400 watts. At idle, your system will draw much less than 400 watts. Higher efficiency power supplies require less energy and therefore cost less to operate. What's more important is to use a power supply that's as efficient as possible for your power requirements.
Just be sure to get a unit that has a high efficiency rating (80+ rated preferably) - The efficiency rating is what determines how close to the actual power need the unit will draw from the outlet so a higher eficiency rated PSU will use less of an abundance of power that winds up being turned into extra heat.
Corsair and Seasonic are two of the brands that have a reputation for high quality power supplies that consistently earn high marks in technical reviews. They are reliable, stable, and come with a 5 year warranty. Some of the newer models come with a 7 year warranty.