Try an on-line PSU calculator, ( via GOOGLE ). Allow about 15% over the top for power surges, such as at system power up, and also a small amount for the fact that as components age they tend to draw a bit more power. If you look at a PSU's specification, remember that the total power output is split into several different levels, ( sometimes referred to as 'rails' ), for different parts of your system - pay particular attention to the +12V figures, as that is where most of the key components will draw their power.