Henry Chinaski said:
The rails are basically well regulated transformers that convert domestic current into the voltages that your PC can use. In other words, the PSUs convert the conventional 110V or 220V alternating voltage from the power grid into continuous voltage used by the PC electronic components, which are +3,3V, +5V and +12V.
Power (Watts) is a factor between current (I) and voltage (V) (P = I x V). If, for example, the transistor is operating at 12 V, the maximum power provided by the Corsair CX400 would be 288 W (12 V x 24 A).
Anyway, the semiconductors and the inductors have a physical effect calling "de-rating" and because of it they lose their capacity to deliver power with temperature, therefore a maximum power measured at a low temperature may not be reached when temperature is higher. The manufacturers of quality PSUs guarantee that their PSUs can deliver their labeled power at 40°C to 50°C, so those PSUs will deliver their labeled power under a real world scenario and not only at the manufacturer lab. This is an important aspect to consider when we want to buy a PSU.
About the power distribution, nowadays a typical PC pulls more power from the +12 V outputs because the microprocessor and the graphic card, which are the two most power hunger components from the PC, are connected to the + 12 V outputs (apart of the hard drives, optical drives, some fans, etc which are connected to it too). And that's why, as shubham1401 has said before, the 12V outputs are the most important ones. Nowadays the 3,3V and 5V outputs are responsible for the mobo, memory, expansion cards,...
If you see that a PSU can deliver more power from its +5 V and +3.3 V outputs than from its +12 V outputs, it means that the design of that PSU is outdated. Some years ago the power distribution was different and the PSUs was maded in order to work properly with that hardware.
That said, I recommend you to go for one of the following PSUs:
Antec EarthWatts 500W (EA500-EC) (by Delta Electronics)
Corsair VX450 (by Seasonic) (the CX400 is enough for your system, but paying a few more bucks you will have a better PSU with more power if needed in a future).
Seasonic S12II bronze 520W
Enermax Eco80+ II 500W
You said the cx400 only has 24amps on 12v when it has 30 amps which is the most powerful 400 watt unit you can buy. So CX400 with 30 amps 12 x 30 = 360 watts of power on the 12 v which is quite impressive for a 400 watt unit and for the OP is the unit I would recommend he gets the most, it would serve him well.