8600GTS uses external power input, meaning that it may use more than 75watt at full load.. to simplify the matter, most of modern computer hardware uses +12v rail.
Watt (W) = Voltage (v) x Current (A)
Let say your card does use 75watt and fully utilize +12v rail. So it will need 6.25A from your power supply at load.
At your power supply, there are power-rated specification written on it
It will be +3.3v, +5v, +12v, and so on.. Take note that (in cheapo power supply) Max Output is the max current (or wattage) output for just a limited period of time (normally 30-seconds). The continous power output rate will never reach those stated at power supply (unless it is reliable power supply).
You also need to calculate the power usage of your processor, motherboard, hard-disk, optical drive, RAM, other add-in cards and so on..
Edited by fenzodahl512, 11 February 2008 - 06:47 AM.