NVIDIA offers CUDA for the 8800 GT and on mikes benchmark scene I found some GT 8800 entries with CUDA times.
So at one point it did work. Does it still work with Cycles?
I am OS X and got the offer to get a Mac Pro 8 core extremely cheap but it only has a GT 8800 in it.
I was also curious if maybe a 560 could be put into a secondary PICe slot simply to be used only for GPU with Cycles while the 8800 GT runs the display.
If all fails I would also be ok to do the rendering on Win and I hope that at least there the 560 will work fine or will there be any boot issues as well??