I wish we could get an idea about the 1.6 & 1.8 G5's though. Sure the dual 2.0 is wicked fast, but where do the other two fit in? Obviously I'm one of many who can't afford the top o' the line. But knowing where the singles are rated would help answer a lot of questions. Like, should I just buy a dual G4 instead?
If you must have expandability for up to 4 drives, an extra optical drive bay and the ability to boot into OS 9, the last of the dual G4s is what you want.
If you want to take advantage of the latest technologies; faster processors, faster I/O, future hardware support and suffer the possible exclusion from applications written particularly for the G5 or hardware devices specifically configured for the G5, the choice is obvious.
I can speak from experience that purchasing non-current hardware generally leads one to an early hardware upgrade path, which can become as expensive as having bought the newest-latest-greatest at the time.