All of the power ratings listed for GPUs are for the system as a whole. They generally test worst case scenario using a top of the line core 2 quad extreme edition CPU with a massive power draw. A general rule of thumb, a good 550W PSU with ~40A on its 12V rail will run any card, a good 750W PSU with ~60A on its 12V rail will run any dual card setup or a dual GPU card.
While the GXS 700W isnt great for a 700W unit, its more than enough for any single GPU graphics card.
I also agree with going for the 5850, its a bit cheaper some places, uses less power, and therefore generates less heat, i wouldnt necessarily wait for the GTX470 though as thats over a week away and unlikely to be any cheaper, nVidia has a habit of higher pricing than ATI.