how to boost my 120mm case fan?!

im looking to hook up 4" ducting and a 120 mm case fan at the top of my 2x4x5 grow tent to cool my 400w hps bulb in my yield master II - 4" reflector w/ glass (sorry no pics! camera is broken). i heard you can use cell phone chargers to power them but a/c adapters or laptop chargers put out a lot more power...so i looked around and found a 90 watt a/c adapter. would that be too much power and possibly fry the fan motor? if so whats the best amount of watts to get maximum air flow w/o overdoing it

the important thing is the voltage. the fan is probably 12V if it's a computer case fan, but check if it says. You can use any DC adapter that is 12V to power the fan. The should also be a max current listed on the adapter (i.e. 700mA). One fan should draw 400-500mA at most and i doubt you'd find any DC adapters that low power. Just wire same colors together on the fan wires and adapter wires, solder or twist on wire caps to connect.

Alternately, if it is a computer case fan you could just use a computer power supply and then you could add even more fans later.