Since you mentioned it, I've gotta ask.
What's with all the modern 700W+ PSU's if no computer ever uses them, even under load? Is it some kind of burst wattage required on bootup to fill capacitors and whatnot?
First let's deal with the, "burst wattage on bootup' question. Audio amplifiers such as a home AV receiver do this; The speaker protection relay is set to not supply power to the speakers BEFORE the power supply capacitors are fully charged. These caps are very high capacity, because transistors are basically low voltage, high current amplifiers. The PSU caps remove the line "ripple" and prevent the voltage from dropping when a huge trasnsient spike in signal occurs. But the overall capacity of the PSU is not increased to accomodate this. That there is no power supplied to the speaker and final amplifier before the caps are charged is to prevent damage to these parts. The sudden onrush of current to discharged caps would fry the transistor junctions. This is the click you here when your receiver finally has speaker output. I haven't heard any such click from the PSUs in computers I've dealt with, so let's move on.
Of course trying to use a 250 watt PSU in a high performance gaming rig is foolhardy if not ridiculous. But manufacturers of video cards always give system PSU recommendations right on the side of the box. They usually read like this, >"system requirements"<. So now, if the box says, "400 watts minimum", do you think that they actually mean you need an 800 watt unit? Not bloody likely. Methinks a > GOOD QUALITY< 450 would do the box proud.
Many thousands of words have been written all over the web, and right here at Techspot, about brand, quality, and capacity of PSUs. So we've reached the point here where we begin to reinvent the wheel.
Then there's marketing, if you're stupid enough to buy a 2,000 watt PSU to power a 500 watt computer, no one at the store is likely to stop you. Look to receive encouragement instead. Be the first kid on your block to have one, that'll be $300.00, cash or charge?
All things being considered, every recommendation has to be tempered by what you have as a computer, what you would like that computer to become after upgrades, and how much you have to spend, and that's why every thread PSU thread is slightly different. The are PSU estimation tools around the web, Newegg comes to mind, and the suggestions here from experienced users. At the end of the day, only you can determine if you're a 4 15" subwoofer kinda guy, or you would just like to get the sound system right.