Many firms are selling 1000W power supplies right now but even for high-end systems that's still overkill. FUD Zilla reports that a Core 2 Duo system with ATI's upcoming Radeon HD 2900 XT graphics card runs fine on a 500W PSU from SilverStone.
Our sources confirmed that the Radeon HD 2900 XT works great with a Silverstone element 500W. This PSU has two power rails and each can provide 18 Amps. So this should be enough and the card works well under these conditions.
This PSU has two separate rails for the graphic card and it is doing the job well and it can power the Core 2 Duo system at the same time.
However, if you want to run the card in CrossFire you may need a 750W or higher power supply.