A 400-watt power supply is rarely adequate for powering anything more than a low- to mid-range graphics card, but this also depends on the quality of the power supply. For example, a low quality 500-watt power supply will actually provide less power and efficiency than a high quality 400-watt supply. A good quality power supply is about 80 percent efficient. It is an essential system requirement to take note of when you are building a computer or upgrading an existing one, since using an inadequate power supply can cause system instability or even damage to your components. Most graphics cards also require one or two 6- or 8-pin internal power connectors.
Older Generation nVidia Cards
A 400-watt power supply will easily power almost any graphics card from the older nVidia GeForce 6 or 7 series. With some of the less powerful models, there will even be some headroom for overclocking. Most cards of the 8 and 9 series will also run, except for the "Ultra" or "GX2" versions. These cards should not be overclocked if running on a 400-watt power supply.
Newer nVidia Graphics Cards
Unless you have an exceptionally efficient PSU and/or a very undemanding processor, the only newer nVidia graphics cards that will run safely with a 400-watt power supply are the budget range models such as the 430 and 440 series. The higher-end GeForce 460 uses less power than most other cards of the mid-range line, and you may be able to run this provided your processor is particularly energy-efficient.
Older Generation AMD Graphics Cards
Any older low- to mid-range ATI graphics card will run with a 400-watt power supply. This includes products from the 1, 2 and 3 series. Most graphics cards from the 4 series will also work, except for the dual-core "X2" versions.
Newer ATI Graphic Cards
Any card from the 5 series should work, with the exception of the 5800 series. The same applies to the 6 series, where anything less than the 6700s should work.
Integrated graphics have very low power requirements since they have slow processors and share system memory. This is one reason why integrated graphics are predominantly used in laptop computers. A 400-watt power supply is ample for any integrated graphics chip.