How to build a Computer - 006 - Choosing a Power Supply Unit
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In short, Think of the future. are you going to be upgrading the system? if so buy more than what you need.
750-900Watts is usually more than enough for most people.
A Modular Power supply is a smart idea if you don't want to have to deal with WIRES EVERYWHERE!
Get the power supply that is for your case (ex: ATM micor-ATX, etc..)
Lastly, get enough amperage on the +12V Rail to handle your Videocards n such... Anything over 40 is good!
Good Luck!
Try looking at:
TigerDirect
NewEgg
NCIX