The R600XT, or Radeon HD 2900 XT needs two power connectors, one 2x3 pin and one 2x4 pin. If your PSU doesn’t have the 2x4 pin one, then you can plug in a 2x3 pin one, but you wont get the full potential of the card.
You need the second 2x4 pin for overclocking, as othwise ATI's Overdrive overclocking utility won't work at its full potential. You can read more about it here.
The card will overclock even without the 2x4 pin connector, but the scores won't be great. We heard that there will be a lot of new PSU's introduced starting Monday. Here is a diagram of how it looks in the real world.