As a longtime mac (and before that, Apple II) user, I've noticed a rather simple pattern for machine obsolecense, and it's helped me make efficient purchases.
I usually buy the "second-best" machine available - because usually that incremental increase in power is relatively expensive, but the second-most-powerful is a good value. This would be akin to buying the PM G5 Dual 1.8 now.
Soon after a new machine is released, there'll be another released that's incrementally better. You have to be okay with this. It usually takes about 2 years before you notice significant performance differences compared with the latest machines, but the machine you bought is still very usable. At around 4 years, the latest software tends to not work so well (or not at all). At around 6 years, you can still use it for basic purposes, but any recent software innovations are unusable. Thus, I usually buy at 4-year-intervals; 2 year intervals if I really need to use something that's just come out; 6 year intervals if times are lean.
Right now I have a Pismo (400Mhz G3, 3.5 yrs old) which is adequate for photo editing, internet use, word processing/spreadsheet/drawing, etc. I've upgraded the RAM & hard drive, which has extended its life, but video editing is slow, and some apps (e.g. Garage Band) won't work. As this was a particularly well-designed powerbook (ergonomic, support for multiple monitors, etc.) I will probably be hanging onto this for awhile and getting a desktop system.
It doesn't seem to make alot of sense to get a single-processor G5. There's a tremendous difference in terms of performance between single & dual-processor systems. True, eventually you'll see the 64-bit processing coming into its own, but probably not before the machine becomes modestly outdated. The machines that seem to be the most logical purchases at this point are the 15" powerbook, the eMac, and the PM G5 1.8DP. Though they may come out with a faster machine tomorrow, I doubt that the above paradigm will change much.
--Dave