Ok, still looking for answer if Corsair VS650 would be good to power 1 S3? I am not good with that stuff, so I will not be modifying server PSUs.
For 1 S3 you should be fine. Even over clocking it it shouldn't pull more than 450 watts, which is about only 75% efficiency. Most PSU's can handle 75-80% efficiency without issue, and Corsair has good products in general.
Ok good, what about the cables? Is it ok if 2x 6pin connectors are on the same cable?