Power Supply

Power supplies all come rated at different wattages. This is a rating of how much power they can deliver to the different components of your computer. A good starting spot is to use an online PSU calculator to obtain a ballpark estimate of what wattage power supply you will need. Below are links to two different calculators that should work well.

http://educations.newegg.com/tool/psucalc/index.html

http://extreme.outervision.com/psucalculatorlite.jsp

After you've gotten your estimate from the calculators I would recommend adding at least 50 watts to the given rating. Then, purchase a power supply with at least that wattage rating. When buying a power supply the biggest thing to consider is quality. Antec, Corsair, and Thermaltake are all pretty good brands to pick. If it's built with cheap parts then you can plan on replacing it in a couple years when it blows out, although if you're in a pinch for money it's still not a bad option. The best way to determine quality is by reading reviews for the product.