Well I just ran it for 5 hours straight without the fan plugged in and it didn't any hotter than it would with the fan plugged in. If I could only find a 6GB hard drive that is quieter. The current one is quite loud when spinning.
You have to take half the damn iMac apart just to get to the power connector. Good thing those iMacs are easy to work on.
Extend the wires for the fan, and attach them to the hdd +7v rails (red on the fan on yellow on the hdd, black on the fan on red on the hdd). Requires some soldering but it'll solve any of your noise issues.