Manufacturers are nowadays bound to their respective marketing gurus resulting in ridiculously exaggerated trends which the usual customer tends to overlook. Let us talk about power supplies for instance. With 1000 Watt power supplies available from many companies and 2000 Watt power supplies just around the corner one can come quickly to the conclusion that you really need that much power.
But you don't! Mostly every website out there who puts PC hardware states the electric power consumption or gives a rough figure about it. If you then add those numbers together for every component you have you might be surprised about the result. For Example, the guys from Legion Hardware put together a dead fast machine consisting of a quad-core Intel CPU, two GeForce 8800GTX cards, a feature-packed ASUS Striker Extreme motherboard and four 1GB DDR2 memory modules. Additionally the system had four Seagate 320GB hard drives and six 120mm case fans and let's not forget the water-cooling system. Now reckon what wattage the power supply they used to power that beast was rated at!
550 Watt - They took a Thermaltake Purepower 550w and it works like a charm for more than two months now.

Power supply output ratings are actually quite inaccurate and can be very misleading. This is why it is very important to purchase a trusted and respected brand name.

If you came this far I would suggest you reading that article, it's well written and outlines every aspect of what to have in mind when purchasing a decent power supply.