OK when looking for a PSU there are a few things you need to look at. AMPS on the 12v rail, Build quality, Energy rating and Wattage rating.
Lets start with wattage rating of the PSU. When using dedicated GPU's they have a minimum wattage rating, as you already know, but this number is a recommendation for the hole system and not just the GPU. The GPU manufacture sets this level to help guide us so we don't buy the wrong parts for our system. One thing they do not tell you on there official page is the amperage the card will need on the 12v rail. This makes it hard for us to judge if a low quality PSU will run a new release card. After web sites get there review cards they do the tests for how many amps the card uses.
Amps on the 12v rail is one of the most important parts to watch for when purchasing a new PSU. here is a little reading to help clarify the differences in amps http://en.wikipedia.org/wiki/Ampere, watts, volts and ohms.
http://science.howstuffworks.com/environmental/energy/question501.htm If your amps are to low there will not be enough current flowing through the system to power all the parts that need to run causing many issues and could be hard to track down for a novice PC builder.
The Energy efficiency rating is a rating on how much energy is lost in the conversion from A/C current to D/C current. A PSU is just a big power converter. Any time you convert electricity to a different format a/c to d/c or d/c to a/c there is a loss of electricity while this happens. They use the Energy rating to let us know how efficient the PSU is in the conversion process. The better it is the more money we save on the electrical bill every month. A few extra bucks for a PSU can be paid back in about 2 years depending on the amount extra paid and how much it is run.
Build quality is a major factor in how the PSU performs and how long it is expected to last. A PSU can be used in multiple systems so purchasing a good one to start with is essential. You can even look at it like the human body, it is the heart of the system, without a good heart your not going to run very long before having issues. What is the difference between a cheaply built PSU and a quality one? The main difference it the quality of the parts that are inserted. Cheap capacitors, cheap chokes and poor voltage regulation will lead to an early death of the PSU and sometime other parts. one of the main things that burn out in a PSU is the capacitors. Cheap caps will burn/blow out a lot quicker than those of a higher quality.
Here is a list of some brands that have good build quality ( there are others but this is just off the top of my head.)
XFX
Seasonic
Corsair
Gold and Platinum rated Rosewill
Antec
look in to these brands and do some studying (when the kids allow) so you can make the best informed decision possible.
How to know how many amps on the 12v rail? On the side of power supply's there is a label that will give this information. It will tell you how many rails there are if more than one, how many amps are on each different voltage supply and how many watts for each also. Make sure you research the PSU your interested in to make sure the values are what you need and not lower. Once you make your decision you can come back here and ask peoples opinions to help keep you from making a mistake.