This is somewhat true, but all you need is 2 pci connectors into the s3 for 450+ ghs on stock. I have 4 s3s on one 1300 evga that's pulling between 1800 and 1900 ghs. But please remember also in just auctioning this so no matter the price there will be a winner if you even get an s3 for 100$ your roi for just the machine is about 3 months.
if you have 4 S3's connected to one 1300 watt PSU then you are really pushing the limits of the PSU. I don't know exactly what happens when you push the capacity of a PSU but that's not something that I would do. I was under the impression that three would be a better limit of number of miners to run on a (gold) 1300 w PSU (assuming the miners are S3's and not something that uses more electricity then an S3).
I would also point out that your ROI figure assumes free electricity which will most likely not be the case for most people.
