Well there's two answers i have to this question.
1) It depends what you want to use the machine for, if this is a question of whether you should upgrade or not i would stick with the 2Ghz imac with 4gb of ram. because the speed increase would not be very noticeable in reality.
2) Technically the new imac will be faster however if we are comparing the amount of ram the machine has then the processor will be faster but lessening the amount of ram in the machine will dramatically reduce the amount of applications you can keep running at once and multitasking you can do.
The Speedmark rating for the two machines in question would probably be about the same unless they both had 4gb of memory in which case the new one would score a bit higher.
I don't have any exact figures but i have used both the new 2.4ghz black imac and the old 2ghz imac (as it was my old machine), i doubt you would need the upgrade unless your current machine is struggling with the work you are doing.
Hope this answers your question, sorry for the verbose reply. 🙂
