The new 1.6 G5 rocks! My previous machine was a 867 g4 which was not slow at all. I was actually quite impressed with the speed bump, especially in photoshop. Now, to be fair, the 867 only had 640mb of ram while the G5 is running with 1.25 gigs. My immediate disappointments are my inability tou use any of my current PCI cards or use any of the numerous zip drives I have lying about.
As for dual processors, I administer about 20 macs now. The 3 machines that I've had the most problems with are the only 3 duals we have. I have never quite felt any kind of speed difference in these machines from their single processor counterparts.