According to this :
eXtreme Power Supply Calculator Lite v2.5, it can. Coming from one GTX260 retailer, minimum PSU requirements for one card (for the whole system) is 500W and the GTX260 uses a maximum of 182W.
Considering the GTX260's are power hungry, I'd aim for a 800-850W, leaving enough room for further expansion and overclocking.
Lowest PSU on Nvidia's SLI web page for two GTX260 is rated at 800W. But that's bollocks if you ask me, considering they recommend a 1300W unit in the same category.
Anyways, just make sure your PSU is SLI ready and has the necessary connectors.
At last, try making sure you get revision G0 for your Q6600, as it's usually a superior revision (less heat, thus easier to overclock).