Is it possible to disable the discrete GPU(s) without physically fiddling with the hardware, and run the system exclusively on intel sandy bridge GPU - in order to save power? Can one do the switch without rebooting? Which desktop motherboards support this?
Yes. That functionality is provided by Lucidlogix's Virtu and it's for motherboards with H61/H67 and Z68 chipsets but I think it also requires support in the BIOS so check any specific MB your looking at.