You definiteloy did not make a istake buying that. That is one of the best PSU's out there. I own the OCZ520ADJ (not the SLi), and it's the same unit. You got the better one though becasue it has full support for SLi without the adapter. Mine has a green fan, and yous will have a blue fan.
You will love that PSU! It is perfect for OC'ing, and it will power even the most power hungry systems. IMO, that PSU is the best. When you take into account the quality, and all the features you get with that unit for the price, it cannot be beat.
It is also a geaorgeous unit. The finish on it is a Titanium Chrome looking finish. Be sure to keep your window clean. This one is a looker. You'll be proud to show that PSU off.
The rails will be solid as a rock. If you measure them with a DMM under load, you're gonna be shocked to find that they hold their voltage no matter what you load the system with.
A mistake? NO! Just the opposite. You made one of the best moves ytou could make my freind.