Where do I hook the fan up to.
Also with this over power the psu with adc monitor hooked up as well?
There's nowhere to connect the fan on a Digital Audio. You need to build a custom connector to rig it up to a 12 V line from a hard drive or somewhere.
As long as your PSU isn't too heavily taxed by other components (PCI cards, drives, etc), you should be fine. That being said, I've got my DA really maxed out (4 HDs, DVD, zip, vid card, Tempo Trio, dual 1 GHz G4, etc). That's really taxing my PSU, and I can notice it.