Gedon:
"As for turning it on, you just need to connect Pin 13 on the main connector (usually its green) to ground. That is a sense line so its very low voltage basically no current. You can even cut off the main PSU cable except for those two wires and put your power switch on that."~ "As for turning it on"... turning on the computer PSU?
~ Connect Pin 13 and the main connector (usually green) to Ground?
Yeah, sorry I wasn't more clear. A standard computer power supply will not turn on using the switch in the back (if it has one at all). Instead, they use a sense line to determine when the PSU should turn on.
See this image:
I think in this case a picture is worth a thousand words. It's just jumpering these two pins with a small piece of wire. You can do it with a paper clip or whatever you have at hand. Like I said, the current is very low, as is the voltage. Just jumper the two wires (the green and the black) and the PSU turns on.
As for the current needed for your servo, I am seeing a variety of numbers quoted. My WAG is that it would be max around 500mA each. So you would need 2.5A. This is EASILY provided by a computer PSU.