Looking at Intel's Processor Finder
, the i5 in question is the i5-750 (actually the only i5) and the i7 is the i7-860 which is somewhere between the i7-920 and the i7-870 (obvious, eh? - not!).
Looking at these processor specs, the i7-870 is obviously better than the i7-860 by 0.13MHz. Looking at the performance between these two i5 and i7-920, on x264 encoding, it's approx. 28% faster, on the RAR operation, it is more like 9% faster. Naturally the i7-860 should be slightly faster still, approaching the i7-870's results.
Looking at the processor finder again, the E8400 is probably slightly quicker than the Core2Duo in these new iMacs, because it has 1333MHz front side bus, while the E7600, is the 3.06MHz chip with a 1066MHz bus. Also, it has only 3MB L2 cache, compared to 6MB. The E7300, is the 2.66MHz flavour of the E7600, so performance will be better than that chip but probably worse than the E8400.
As Munch said, there's no competition based on these two benchmarks: an i5 or i7 is going to 'smoke' a C2D.
For those wondering i5 vs i7, hyperthreading is going to help where you are CPU limited. If you are GPU or RAM limited, or I/O limited, having the HT tech won't give a noticeable boost, but most applications are approaching multi-threading aware, especially high end editing and image processing where the maths involved are intense. Being able to process in parallel complex operations will benefit with more cores, virtual or otherwise. With respect to a real/virtual core, you have a single hardware core say and the running thread can stall, having HT means that another thread can pick up and use the hardware core with a lower hit for the context switch of the thread.
Just my two peneth, hope it helps.
Edit: aah I see more benchmarks, my comments above refer to the first two :o)