Electronista has benchmarked the new Quad Core i5 chips in the new iMac, and comparing his scores to mine, its pretty clear we've got almost 2x some scores in some CPU/memory tests.
Specifically, using his charts and mine, it wasn't hard to recognize the jump in the multithreaded, 64 bit results from geek bench in the categories of integer, floating point and memory streaming tests, as well as the threaded tests. (Memory tests were slightly faster, the others were drastically so.) Interesting, as the Core i5 chip is clocked at 2.66GHz and the Core2Duo iMac I tested runs at 3.06GHz.
(The turbo boost function, which overclocks the Core i5 chip to up to 3.2GHz when running non-multithreaded apps, should be kicking in performance here, too.)
Interesting, but two things to remember: Core i7 chips are coming out for the iMac shortly and will run at 2.8GHz and have hyperthreading so the 4 cores emulate 8. And there are still not many (if any at all) major OS X apps that can take advantage of Snow Leopard's multicore support. [Electronista's tests, Gizmodo's iMac Review]