You all know the story of two R600 power connectors and the 2x4 pin one is for overclocking. The R600 consumes 240W at default speed when it is doing 3D work and this is a lot of power.
If you try to overclock the card it goes way over 250W. From what we heard it is getting closer to 300W and this is just ridiculous for a graphics card.
Two of them in Crossfire will consume as much as 500W+ and you will need a lot of power. A good 700W PSU is enough for a single card and a modern CPU, but you will need much more for two cards.