A VelociRaptor in a 2006 iMac will probably FRY!
The drive noise (mounted behind the screen) will also be big-time annoying.
If you still want to try it, this might be of use http://jasontomczak.com/2007/11/23/4/
The VelociRaptors run remarkably cool actually. The 2.5" form factor with the attached 3.5" heatsink/mount works really well. And I believe the iMac's have a slight amount of cooling for the hard drive provided by the intake of air before it goes through the CPU fan. I run a pair of the in a RAID-0, mounted in a drive cage, with minimal airflow (due to crappy case design) and they barely break above ambient air temperature.