What You Should Buy
If you're not running a video card at all, then buy a low-end Seasonic power supply and call it a day. For the vast majority of systems, a 300 watt power supply like this Seasonic is actually overkill. Plus, Seasonic's power supplies are backed by three-year warranties, and even their less powerful ones can be had with efficiency ratings of 80%.
Should you want to use a high-end video card, then Corsair's 450 watt power supply is an excellent choice. And if you're really outrageous, you could spring for Corsair's 650 watt model. It may not impress nerdly friends as much as a 1000 watt behemoth with five colors of LED lights and a fan that could cool a warehouse, but it will run virtually any combination of hardware.
Whatever you do, be sure that the power supply you buy is from a quality brand. Corsair, Seasonic, Antec, Zalman, Silverstone, and PC Power and Cooling are some of the better power supply brands. They are more expensive than products from no-name brands, but they are backed with excellent warranties and won't fry your computer if you put them under more load than they can handle, which is often half of what they say can they handle.