Use something like this to calculate the necessary PSU wattage : http://extreme.outervision.com/psucalculatorlite.jsp
Then add about 20% for safety margin. Stick with Quality PSU providers such as Corsair, Seasonic, XFX, OCZ... and check to make sure that the PSU you chose has enough connectors for your use.
Stay far away from the el-cheapo brands. $20 bucks for A 650W PSU is a good indication of an el-cheapo PSU. Most of those will barely work at half of their rated wattage and they often don't include the more recent connectors needed for your components.