Let's use NewEgg's website, for example: each component you plan on using will have a 'specs' tab, and you'll see the power consumption/wattage listed. Write them down for every component you're going to use in your build, then total it up. If you plan on future upgrades (like a 2nd video card), add that in your total. Then round up a little. Don't forget to add the wattage for your case fans and any blinky/flashy lights you might throw in your case!
For example: if your total wattage is 510, i'd recommend a 600 watt PSU. If your total is 550, i'd play it safe and buy a 650.