If you CTO the stock 24" with just a CPU upgrade then just 400Mhz is the technical answer.
I think for 9/10 tasks it will make little real world difference right now but I bought one because I wanted to keep it for at least 3 years and figured sooner or later you will come across a bit of software where it will really matter. I figured that another 6 - 8 months of durability was worth the price and I get to own the fastest imac to play with now.
Edit: Oh and one last thing if someone writes an overclocking application for OSX (must come soon) then I have a CPU which is pretty much designed to cope with it. So who knows it maybe a 3.2 GHz machine at some point.