How exactly do you choose a power source when building a computer? I am a total noob so I have been doing so studying but I can't find much on power sources. How do you choose wattage? What are brands to stay away from? Is there any reason to buy one of the higher priced ones? Thanks for helping out a newb, if there is already a thread like this, fell free to let me know.

I could sell you a couple that I don't think work for pretty cheap if you want 'em.

Start with a reasonable estimation of the power you need.
http://www.extreme.outervision.com/psucalculator.jsp

It is important to note that not all PSUs are created equal. Many cheap PSU's push the Total Wattage up by increasing the amperage avaliable in pretty useless areas such as +5v and +3.3v.

Also, check with your mother board, CPU, and Graphics Card manufacturers to see if there are any special guidelines for PSU selection.

For example, when I was constructing a dual processor system, the manufacturer stated on their website that it could be powered by a standard ATX PSU. However, upon further investigation in the User's Manual and online forums, only 1 ATX PSU sold was able to supply the 36A on the +12v rail the processors required! And that does not include the extras such as HDDs.

I would reccommend PSUs from Enermax and Antec. I have used Antec ATX PSUs for many years in a number of systems. I am currently using SSI powersupplies from Enermax. SSI are server grade PSUs, not directly ATX compatible, but many come with adapters. They usually provide multiple +12v rails to provide large ammounts of power needed by Video Cards, HDDs, and other devices.

Search the internet for information on your selection of CPU, Mobo, GPU(s), and after finding a PSU that looks sutible, add it to the list. Many times there can be quirks with one particular model.

It would be best to consider the PSU the investment piece of a new computer and as such, should be of the highest quality that can be reasonably afforded; even if it means leaving somthing else out for a bit. RAM only gets cheaper, but a problematic PSU will kill the whole system.

If your buying one online, go ahead and look at the http://www.newegg.com reviews and go for the ones that have alot of positive reviews (4+ stars, 20+ reviews)

That'll save alot of headache.

Don't necessarily be afraid to buy a lesser known brand, however. It's already been said that not all PSU's are created equal, and that's true, but I've had just as much success getting a no-name brand with specs equal to those of much higher prices.

I'd check your voltages under load though, low cost supplies typically use inferior components and typically don't hold their lines nearly as well as more reputable ones. Most likely won't be a problem unless you're right on the envelope for the rated amperage. You can use them, just don't push them.

I have an OCZ GameXTream 700w and am quite happy with it. I have an Opteron 165 (dual core) with a 900MHz overclock, a 7800 GT, 7 120mm fans, a 92mm fan on my CPU, and 6 hard drives, and my 12v rails never drop below 12.2v (using a multimeter).

Personally I've had good luck with Antec's Truepower and Smartpower series.