Power consumptions differ from system to system since each PC isn't created equal.
Your main priority when choosing a videocard is complying with the +12 volt rail. Make sure it can supply enough amps in corresponding videocard requirement. If you don't know your +12 volt rail's rating, take your case off and check it. The graph is usualy located on the side of the PSU.
Hope I answered you Q, yet you really should elaborate a bit about the problem.