The reason for wanting a 80+ rated PSU is that on a non-80+ PSU you might typically have 65-75% efficiency. This means while the PSU is rated for eg. 500W, when it delivers those 500W, it'll draw (500/0.75) = 666W from the power socket. That's 166W turned into heat right there!
If you game eg. 20hrs a week (or ~1000hrs a year) that would be 166kWh wasted..
If you had used eg. 80+ Silver (85% efficiency @ 100% load), you would only have wasted 88kWh.
80+ rated PSU's have atleast 80% efficiency and obviously the metal ratings means better efficiency Titanium > Platinum > Gold > Silver > Bronze > "White". Titanium
Depending on how long you plan using yor PSU for, remember that it will also slowly decrease in power output capacity over the years.. if you're looking at using it for eg. 5 years, you will only be getting ~80% out of a PSU compared to when it was new (in a somewhat bad scenario, with less degradation over time by more expensive PSU's as they DO use better components, most of the time).
If you watched the YT vid, you'll ask yourself how much power will MY system need? His setup was pretty great - OC'd 4770K, OC'd GTX680, watercooling loop, 1 SSD, 2HDD's, 11 Fans, 2 LED strips (and a partridge in a peartree!) and yet maxload was ~360W from the power socket.
As he stated he had a 750W 80+ GOLD PSU, we can use the table and find that the PSU worked at 90% efficiency and so the system only demanded 324W all in all (and 36W was turned into heat in the PSU).
If you look at the max power rating of the i7-4770K, it states 84W, the 680GTX says 195W = 279W total. If you then add in the mobo, watercooling, fans & hdd (ssd's draw VERY little power) - you get to the 324W total.
So basically, JayzTwoCents could add 2 more 680GTX's and that would still be possible to run from his 750W PSU - even though he'd have to take care and pray that the PSU output didn't degrade too much!
Links to Power Req Calcs
http://www.extreme.outervision.com/psuc ... orlite.jsp
Use the above as a guideline - and use common sense aswell
As a FINAL comment - if you look at the 80+ graphic, you'll see that most PSU's have better efficiency around the 40-75% load capacity and while 80+ Silver/Gold n upwards PSU's aren't cheap, having a little extra means you'll put your PSU efficiency in a better position!
TL;DR - Unless you're doing Triple/Quad SLI/Xfire (or experimenting with voltmodding/hardcore OC) - don't buy more than 800W!!