Over on AppleInsider they have an article talking about how the new Intel iMac stacks up against the previous generation iMacs. They also have a benchmark test between the new Mac minis and the old Intel ones and even the G4 Mac minis.
Obviously the newer machines did better on the tests, sometimes up to 39% better. The new 24” iMac is a really powerful computer.
BareFeats.com also has some benchmark tests on the new iMacs.