You need to think of a power supply as multiple individual supplies. So you have a 5v line, a 3.3v line and 12v line.
Each line will have a maximum rating and it looks like your 12v lines do not supply enough power.
Power supplies are always the last in line to be upgraded, but in a sense, they are actually the most important component of all. When you start using higher end graphics cards, it is essential not only to have a power supply that can provide enough power, but also can deliver it in the highest quality. That is why genuine quality power supplies are relatively expensive.