If you're planning on using two graphics cards in your desktop PC, and perhaps even try your hand at overclocking those along with your processor, it's advisable to get a PSU of between 750 and 950 watts. There are a huge number of these on the market, so it's difficult to know which one is best for your needs. Hardware.Info tested 28 power supplies in this category, to separate the wheat from the chaff.
Earlier this year we tested 43 power supplies between 500 and 700 watts. This type of capacity is plenty for the majority of PCs, especially if you'll only be using a single graphics card. Once you begin using two video cards and start to overclock, you will quickly run into the limitations of a 700-watt or lower PSU.
Most current processors have a TDP of about 95 watts. TDP stands for thermal design point, which basically is the maximum power usage. High-end graphics cards have a TDP of around 250 watts. So when you combine two video cards, add the CPU, add 100 watts for all the other PC components, you're already at 700 watts. Power supplies are the most efficient when they run at half capacity. When you overclock, even with just air cooling, the power usage of GPUs and CPUs can increase by another 30 to 50 percent, and even more for good overclocking processors. These are all very good reasons for getting a PSU with enough capacity.