I have 2 S3s that are now over-clocked. I'm using a Corsair RM850 to power both, with 4 dual PCI-E cables. I didn't have any luck getting them to run stable at 250Mhz, so I backed off to 237.5Mhz.
Results after 1 day:
Both miners have settled in to an average GH/s of ~478, but there's a lot of variance as you can see by the graph. Still, very pleased with the results and the lack of hardware errors.