The main question has been answered properly, of course. I just wanted to add this:
Im glad to see that KingCody pointed out that you can use a 350 watt ps in a standard system as a lot of people think you need 400 and more watts. I have seen systems with onboard vid, nic,audio, a hard drive or 2 and running on 80 to 200 watts power supplies. Standard vendors like HP, Dell, Compaq, etc use power supplies less than 200 watts on some of their pc's they sell.
The truth is that you can have a ton of stuff running on these power supplies, a lot more than what is suggested by those charts and such. Part of the reason is that you usually dont have all your hardware being used all at the same time. But the other reason is that even 200 watts is fine as long as you dont have external video. These days, though, you can get a decent 400 watt ps at newegg or directron for $25 to $40, mainly up closer to the $40 mark for better ones. So why buy a 200 watt ps!
Last, junk power supplies have very poor to partial to no thermal protection at all. When a good ps goes bad it sacrifices itself for the rest of the hardware in the pc, thats thermal protection. A cheap ps doesnt and so all or part or most of your hardware, like your vid card, hard drive(s), and other parts will get destroyed in the event of a power low/high, short or whatever.
This is the best reason I can think of to get a decent power supply!