I have a server and my security system , modem, router, etc. in a locked closet. I cut holes in the top and bought some fans from
http://hometheatercooling.com
There is also a cold air return that runs directly below the closet so I was able to tap into that as an air source as well. Basically sucks fresh air in the bottom of the closet and exhausts the hot air out the top. Works great and my PC temps are stable.
They also sell a thermostat for those fans, but I just keep mine running 24/7. You can't really hear it at all.