Its good to see questions like the ones you asked. Most new people expect to come here and have all their questions answered and dont expect to have to do any research on their on. You wil probably fit in here just fine if you structure all of your questions the way you did this one. It shows that you are willing to do the research, you just need to be pointed in the right direction. Now to your question.
The most important thing about PSU's is the brand. The number of W's is important, but not as important as the brand. The nasty little secret about PSU's is that there are a ton of generics as well as some name brand that are very misleading in the way that they come up with total output. If you find a 600W PSU for $30.00, then most likely it is crap. The PSU is probably the most overlooked piece of equipment. More and more over time im coming to the conclusion that the PSu may very well be the single most important part of the whole system.
Theses days you probably need to consider at least a good quality 400W PSU, but 500W would be great. I have a Thermaltake 480W, an Antec 430W, and a Sparkle"older model" 350W. All of these are good PSU's. Im no expert, but one thing to watch is the individual rails and how many AMPs are supplied to each rail. Here are the specs of a few rails on my Thermaltake.
+5V/40A +3.3V/30A +12V/18A
The more AMPs per rail, generally means a more stable PSU. Some of the better brands are Thermaltake, Antec, Fortron, and Sparkle. The last two are possibly the best price/performance PSU's. I hope this helps!