For a system using a single GeForce GTX 480 graphics card NVIDIA specifies a minimum of a 600 Watt or greater power supply that has a maximum combined +12 Volt continuous current rating of 42 Amps or greater and that has at least one 6-pin and one 8-pin PCI Express supplementary power connectors.
Total Power Supply Wattage is NOT the crucial factor in power supply selection!!! Sufficient Total Combined Continuous Power/Current Available on the +12V Rail(s) rated at 45°C - 50°C ambient temperature, is the most critical factor.
You may find power supplies on the market that supply more than enough Wattage to run the system. However, some of them lack Sufficient Amperage capacity on the critical +12 Volt rail, which is necessary to properly power the critical components in the system (i.e. CPU and GPUs). This is the reason why graphics card manufacturers overstate the power supply wattage, usually by at least 50 Watts, in an attempt to take into account some of those power supplies that have the weaker +12 Volt rail(s).
A standard ATX12V power supply has the following dimensions:
150mm W x 86mm H x 140mm D ( 5.9"W x 3.4"H x 5.5"D )
The depth is the only dimension that is allowed to vary. Higher capacity PSUs will extend deeper into the computer case.
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What is the size or model of your computer case? Specify the brand and model number of the system if you have a consumer desktop unit like an Acer, Compaq, Dell, Gateway, HP, ..., etc.