I'm trying to build a fan with the Arduino and a PS3 cooling fan, but i cant find the amperage required to power the fan. Do i need a transistor, Op Amp or voltage regulator to drive it?
Measure the current - put a fused ammeter in series with the fan, and supply +12v. You'll most likely need a transistor to drive it.
Operational amplifiers are NOT well suited towards driving high-voltage/high-current devices, in this case, and regulated power to a motor just leads to lots of wasted heat. A relay, however, may be a more desirable choice (depending on the current draw)