Re: 1st time builder
They recommend a 450W minimum based on what they believe to be an average system. It takes CPU/mobo/HDD/etc into account but if you plan on overclocking or are using an unusually high powered CPU your numbers may be different. I think the 450W probably assumes the card's maximum power draw plus an average CPU (probably 130-150W). They may or may not put in some extra headroom in that number but anything over 450 should be fine. Note that if you are overclocking, overvolting, or other performance modding you may increase the power draw of your system closer to the maximum limits or even exceeding the maximum power consumption limits of your hardware, so add some extra wattage if you plan on overclocking. A 580W should be plenty, I have a 650W powering my i7 930 overclocked build with overclocked 5870 and it works fine. The GTX 4xx cards do draw a bit more power than their ATi Radeon HD 5xxx equivalents but it should still be fine on that PSU.
Also note that PSU ratings vary between models. Cheaper PSU's tend to display their peak output and not their continuous output. Peak output is the most power the PSU can withstand during a quick, short power draw surge while continuous power output is the maximum amount of power the PSU can supply for an indefinite amount of time (a 650W continuous PSU typically can take power surges above 650W while a 650W peak PSU would not be able to handle that load and would probably fail if the system were to draw 650W for more than a few seconds).
Generally good manufacturers rate their PSU's by continuous output (Antec, Corsair, Rosewill, etc) but cheaper PSU's are questionable. Cheap PSU's can be a good solution to old computers but they're not for gaming builds.