ATI recommends that you get a 750 Watt or better power supply with two 2x3pin PCIe connectors. Otherwise your Radeon HD 2900XT with 512 GDDR3 memory won't even work.
For optimal performance and Overdrive overclocking you need to use one 2x3 pin PCIe power connector and one 2x4 pin PCIe power connector.
The only problem is that no one has one available today, but we heard that there will be at least a few PSU manufacturers that will offer one. We are not sure if ATI will offer a 2x4 pin to two Molex adaptor, but it sounds very unlikely to us.
Smells like PSU upgrade to most of you who is looking at getting this card and wants to get the most out of it.