Hmmm, let's see, overvolted by 8v on a 12v fan. It's hard to say, but wow will it spin fast! :) If you are just using one fan, you can use a 12v regulator from
RadioShack. Simple three terminal hookup (input, output, common). Since a fan doesn't pull much current, heat should be minimal. Figure out the regulator dissipation by using 8v*fan_current. If the dissipation is greater than 1/2 watt, also use a small heatsink. Note: the little tab is internally connected to one of the leads (output I believe).