on an average computer (i'm over estimating to be safe)
CPU (Full Load)
~100W
Video Cards (Full Load)
~150W x2 = 300
Hard Disk
~30W
3-5 Case Fans
~25W (5W/fan)
DVD Drive
~30W
RAM (10W per module)
~40W
So that works out to max power requirement:
(100+300+30+25+30+40)W
≈ 525W
You want your peak power requirements to be less than 80% of PSU capacity
525/0.8 = 656W
So, you can get away with a 650W PSU if you check and make sure there's no noise or spikes when it approaches full capacity.
Get yourself a 750-850W PSU and you will never be anywhere near full capacity so you won't really have to even worry about it.
If you have to spend an extra $50 bucks, do it, its a good investment and your system will thank you by not dying prematurely.
1kW is overkill unless you plan on getting everything water cooled and need to run a pump running off the PSU
The problem with budget PSUs, like the others have stated, is that it doesn't provide clean power, there's always some noise in the signal and depending on the fluctuations, it can be enough to damage your components. Look up reviews and tests of PSUs you are interested in, people hook them up to meters and set them to draw the max output possible and then measure just how clean the power is. If a PSU can supply clean power at max capacity, it will work great all the time. If a PSU can't, then i wouldn't trust my components to it.
I have a bias for Corsair PSU's. This is the one that I've used when building systems for my friends.
http://www.newegg.com/Product/Product.aspx?Item=N82E16817139006
I have the 850W myself.
http://www.newegg.com/Product/Product.aspx?Item=N82E16817139011