GTX 680 should be the ultimate card in a SG05, since it should fit without any modding, and the PSU should be able to handle it's reduced power consumption.
"Overall the card measures 10 long with no overhang from the shroud, making it 0.5 shorter than the GTX 580."
http://www.anandtech.com/show/5699/nvidia-geforce-gtx-680-review/5
And from Nvidia's own spec pages.
GTX 680
http://www./hardware/desktop-gpus/geforce-gtx-680/specifications
GTX 580
http://www./hardware/desktop-gpus/geforce-gtx-580/specifications
can't wait to see a SG05 with a GTX 680 in it.
"Overall the card measures 10 long with no overhang from the shroud, making it 0.5 shorter than the GTX 580."
http://www.anandtech.com/show/5699/nvidia-geforce-gtx-680-review/5
And from Nvidia's own spec pages.
GTX 680
http://www./hardware/desktop-gpus/geforce-gtx-680/specifications
GTX 580
http://www./hardware/desktop-gpus/geforce-gtx-580/specifications
can't wait to see a SG05 with a GTX 680 in it.