I would go with the 2.4 GHZ, as the speed difference is noticeable, and it won't be louder than the 2.0 GHZ model. The iMac is due for updates soon, so I'd hold off until the new spec comes out. This way, you will have the option for getting the Penryn chip (which runs cooler), or buying the current spec for cheaper. Good luck. 🙂