I have a 2GHz iMac with Core Duo.
I have a 2.33 GHz Macbook pro with Core 2 Duo.
When encoding DVD's via Handbrake, the iMac gets about 60 fps, the MacBook gets about 70.
Otherwise, they are both obscenely fast and I have trouble maxxing them out...
It's not a CPU difference, but the Core 2 Duo MacBooks can be upgraded to 802.11N wireless networking. The Core Duo MacBooks cannot. (At least not via a software update from Apple.) I've upgraded to N and bought Apple's Airport Extreme N. It's fast...
-Todd