Something Fuad and others can shew on.
Our system had an Enermax 1000W PSU.
I have the ATI Tech/Fact sheet here so upto 200W is requirement at stock clocks with 2x6pin connectors.
If overclocking then you need 1x8pin and 1x6pin connected and then it consumes upto 225W.
I hope ATI don't bash me for stating this but I don't want misleading information such as that going around.
Fact is a decent quality 400-500W PSU will run of these cards fine if your running a Core 2 DUO system or Athlon X2 based system.