Don't get me wrong, folks:
The single processor 1.6 GHz G5 is a GREAT machine. I guess I should've been more specific stating that compared to the dual processor G5's, the 1.6 does seem slightly slower to respond.
I've been to my local Apple Store, and did an informal test of the speed between systems. To gauge perceived speed, I quit all applications running (except Finder, of course), and gauged how long it took for each individual system to launch Safari, Entourage, iTunes, iPhoto, and Photoshop CS (almost simultaneously, by clicking on each app's icon in the dock). Both Dual's were, IN MY OPINION SIGNIFICANTLY faster than the single 1.6 model.
So, since when upgrading to a system, for me, I'm looking at the most advanced model I can possibly purchase, so that 6-7 months down the line I don't feel the system is running as slow as molassas.
Purchasing a single processor 1.6 GHz, as opposed to a dual processor 1.8 or 2.0, in light of the specs to be introduced in a little over a month from now, is a mistake. BUT, that's only my OPINION.