After you get the wattage needed, I usually get 10-20% more than what is recommended. If they say 400W, a 440-480W power supply should be considered.
I always use Corsair, Seasonic or Antec in the builds I do - not to say that the others don't make good products, but with buying these brands, you are assured of getting quality in the PSU. Don't skimp here - this is what delivers power to your computer, and saving $10-20 on a PSU can create an issue where your entire investment is fried due to a poor power supply.
Some of these are dated. I avoid CoolerMaster and Thermaltake. They make other computer parts well but not psu.
Stick with Antec, Seasonic, and XFX. PC Power and Cooling is another. Enermax is good but they like multi rails which I avoid. Corsair has some really ggood supplies but you will find that XFX is usually cheaper and they are the same Seasonic made supplies. The CX is a budget series that is probably a Tier 2 or 3.