You know good and well I feel the iMac should be using the low power desk top Peryn cpus.
I'd make it simple.
20" 2.6 ghz Quad
24" 2.6 ghz Quad
24" 2.8 ghz Quad
If you like you can have a dual core 20" at 2.8 ghz.
The mini ought to have 2.0 ghz quad core mobile cpu asa BTO for the high end, IMO.
These machines would give as good performance in most if not all applications and in those able to take advantage of multiple cores it would be superior to what we now have to...