The power supply you pick should be able to sufficiently power all of your components. High-end video cards and multiple hard drives can especially draw a lot of power. Mid-range systems should work well with 500 watts and 800 watts is usually good enough for higher end systems, but we decided to buy a 1,000-watt power supply. We also chose a single rail 12-volt setup, which leads to a more stable power output.
The power supply we bought, the Silverstone ST1000-P, has a special feature known as modular cabling. This allows you to plug in the cables you want to use, and leave off the ones you don't. Using a traditional power supply typically meant leaving unused cables bundled at the bottom of your case.
For our computer case, the power supply mounts at the bottom. There's also a vent, so be sure to turn the power supply and mount it with the fan facing out. In some cases, the fan has to stay towards the inside.
After screwing the power supply firmly to the case, start attaching the power cables. For our build, we'll be using the main motherboard connector, the two additional motherboard connectors, a SATA power cable for the hard drives, another SATA power cable for the optical drives, a power cable for the video card and an older style power connector for the case fans.
Connect each to the power supply and snake them through the case to the different connection spots on your components.
(Justin Meisinger in Boston contributed to this report.)