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.
The good news is that at least in Germany this PSU costs under €80. You can buy it here.
But for Crossfire you need 750W+.