|All the above responses have merit. |
The issue is really about how the card manufacturers dumb down the requirements.
Not all 400W PSUs are created equal and the graphics card may not be the ONLY power hungry hardware. Some CPUs draw quite a bit of power too.
Many PSU manufacturers fudge the numbers and/or supply larger than needed wattage on rails where it is not needed.
In modern systems the +12V rail of the power supply (PSU) is the most important rail. That is the rail that supplies the power to the CPU, graphics, and the drives.
You need to add up the +12V amperage draw and verify the +12V rail can supply the requires power with a cushion for overhead.
Conversion formulas are as follow: A=amps, V=volts, W=watts