8 fans .30A = 2.4A, plus stall current to ovecome at turn on.http://www.digikey.com/product-detail/en/NTD4963N-35G/NTD4963N-35GOS-ND/2305685
I think these will run nice & cool for you.
Use a 150ohm resistor between arduino pin & gate.
Connec the MOSFET between the fan '-' and Gnd. The fan '+' to 12V.
Put a 1N4001 diode across each fan, cathode to +12 and anode to fan '-'; this lets the motor generated "back emf" dissipate in the motor when you turn off the MOSFET.
Computer fans cannot be simply PWMed since they are entire brushless motor controller circuits - you can
vary the analog supply voltage to vary the speed, and the 4-wire ones allow for digital speed control. Also they do not exhibit much stall current (they are required not to overheat if they are stalled).
They key thing when choosing a MOSFET is to get a logic-level one - this is essential, and then
to choose one with a low-enough Rds(on) (on-resistance) so that it doesn't overheat - calculate the
dissipation as I-squared-R. These days there is no excuse not to get a device with Rds(on) below 0.05 ohms.
Do not choose a high-voltage MOSFET for a low-voltage circuit, high voltage devices have much higher Rds(on).
Do not choose a MOSFET from its current rating unless you plan on water-cooling it.
The suggested device above has 0.016 ohm Rds(on) for logic level gate drive, so at 2.4A it will dissipate 92mW,
and thus run pretty cool.