*Disclaimer: Our—and others—review units are not high end machines with Core i5/i7 with high end ATI graphics, so we can't speak for the top end of iMacs. Also, the charts above are not indicative of anything, whatsoever.
The 27-inch iMac we're testing is a nice machine, but the specs—3.06GHz Core2Duo CPU with 4GB of 1066MHz RAM and an ATI Radeon HD 4670 graphics set up—are the stock low end parts for that size. And that chip is the higher end (built to order) CPU of the previous generation. (The graphics situation is weirder. The ATI card here is better than the stock 9400M NVidia setup of the old base 24-incher but not better than the built to order ATI 4850 option; the 4850 remains the top end choice for this generation's iMac, too.) These quick test results show a few changes, but, well, there are plenty of issues that nullify any meaning that can be interpreted beyond "duh". (Longer bars are better in both charts.)
First of all, XBench is just falling apart as a test these days, having being revised a long time ago and taking advantage of zero of the latest OSX technologies. Also, from what we've seen, XBench scores take a hit in the openGL rendering on Snow Leopard, compared to Leopard. The new machine seems slower than the old 2.66GHz iMac we tested last year (With OS X 10.5) in XBench in OpenGL and overall because of Snow Leopard, not the new computer. Plus, Xbench is just really, really old. I don't really trust these results, especially running between two operating systems.
Secondly, we used Geekbench. Geekbench runs in 32 and 64-bit mode in Snow Leopard and has been updated to take advantage of varying number of cores. (It's multithreaded better than most software and not surprisingly given the simple nature of a synthetic bench). As you can see, though, Geekbench only tests core system tests focusing around CPU/Math/Memory performance. And here, the faster chip has the advantage, apparently in 64-bit mode, too. This isn't surprising either and the numbers don't jump off the page.
So, you'll have to wait for us to test Core i5/i7 machines with ATI 4850 graphics and—Apple willing— 16GB of RAM before we can comment beyond the fact that this machine is prettier by 45% than the last generation of iMac.
But, even if we had that machine, the quad core CPUs don't have enough software beyond native Snow Leopard apps to really take advantage of the extra cores. There's always turbo mode, which bumps utilized cores up in speed when software isn't running across all four channels, but you're talking about chips that run slightly slower clocks than Core2Duos, so its up in the air how turbo that turbo can get.
Point being: I have nothing for you. More soon.