300 or 400 watts?

[Dnana]

I am looking at getting a new case and I see power supply units at 300 or 400 watts - What is the rule for deciding on wattage requirements - please tell me what type of system configurations require 400 vs. 300 and whether 300 watts is sufficient. Thanks.

 

[Lokan]

I was told to look for a powersupply with at least 220w on the combine rails for the 3 and 5v. Here's the one I purchased..
Linkah Has free shipping which makes it reasonable and has been running my rig (listed below) flawlessly. I've even added a ton of fans to the new case I purchased and still no issues. Fairly quiet too

Btw: this one is only 360w but is rated better than some 400w PSU's...