The PSU is one of the most imporant parts of your build. Most new builders overlook the importance of getting a good quality psu. If the psu blows up it ussualy will take other components with it creating a very expensive repair build. Things to note about psu's: high wattage does not mean it is a good psu. This is mostly a marketing sheme. The 12 volt amperage is very important.
Heres a breif example of a good quality psu and how to get the amperage information:
1) click the link below
Crosair 850tx
2) go to the specifications tab.
3) under spec, look for output. It should read like the follownig:
Output +3.3V@30A, +5V30A, +12V@70A, -12V@0.8A, +5VSB@3A
the 3.3volt rail has 30Amps, the 5volt rail has 30A, the 12V rail has 70amps ...ect.
Now your probally asking yourself, well how do i know what's an amperage value that will work for me. The main thing that uses the 12volt rail is the vidoe card. So you'll have to do a bit of research and determine how many amps your vidoe card requires.
So for example you have a video card that requires a 33amp 12V rail. So your 70amp on the 12 volt rail will be overkill. But if you hook up two graphics cards, 33 + 33 = 66, now the psu will be getting used much more effecently.
In general the forum favorites i'd say are PSU's made by Antec, Crosair and PC Power and cooling. They are the most resonably priced and provide pretty good quality products.
This also might be helpful: PSU caculator