Don't take it that literal. Modern CPU's can use up to 90 watts (AMD) or 130 watts (Intel), modern videocards do about 80 watts. The powerconsumption of the other components are relatively small.
20A on the 12V is really needed for modern systems. The A64 alone requires a minimum of 12A on the 12V line! Depending on your video card and the other components, you might need another 10A or so.
To juice my system I have a Levicom (German brand, not known but a well performer) PSU with a rated capacity of 550W. It does 35A on the 12V line... I had stability issues with a 12A model. My system? Athlon 64 3200 @ 3500, Radeon 9700 @ pro, 4hdds, 4odds and some fans + lights. Not THAT much eh?