"When clock speeds are identical between systems, such as with the new low-end 2.66GHz iMac and a mid-range predecessor from last year, the difference in test scores becomes almost unnoticeable, coming down to 11 points in the case of the two all-in-ones. Even the 2.93GHz iMac, which replaces a 2.8GHz model, has only a 189-point advantage -- or about a 5 percent speed difference that directly maps to the higher clock rate."
Apple Insider does note that there's probably a big leap in video performance due to the new Nvidia chips inside, but in operations where there's a lot of CPU and memory throughput, there isn't that big of a difference between the current model iMacs and the new ones. That's food for thought if you're planning on purchasing one of the newer systems. A caveat, of course, is that we don't know how much this will change when Snow Leopard rolls around, so choose your weapon carefully!