Primate Labs, the blog for Geekbench, has tested the speeds of just about every Mac in the last 6 years or so up to the present. It's pretty spectacular. My old Quicksilver G4 comes in at 841. This is based on the score of a Power Mac G5 @ 1.6 Ghz being the base at 1000. My new iMacs came in at 2862, more than 3 times faster than the Quicksilver, and they probably used only 1 Gig of ram where I have 3. Tests at OWC show going from 1 Gig of ram to 3 is a better than 30% speed jump.
But it is very interesting to see where everything stands. At the very top is the new Mac Pro Dual Quad-Core Intel Xeons (which means 8 processors) at 8626. It better be that fast since a single one would have cost more than both my iMacs combined, which is why I didn't get one.