Here is my theory; everything in your syst uses power and will need to take that into account. That does include DVD, all flavors, CD, all flavors, MB, all expansion cards, HDDs, the amount of RAM, etc. I personally burned out a p/s not having enough jiuce to power everything in my syst. With the more up to date rigs, you will need to buy a large enough p/s to run everything.
I forgot to add fans also need juice.
This will also include water cooling also.
Your friendly neighborhood pc tech.