In the long run, for a non-pro user, it's a question of throughput vs. latency. My take on it is that a DP machine "future-proofs" your purchase due to the fact that, when the individual processors really aren't that fast (relatively) anymore, you know that you've still more than one processor to handle jobs in parallel. This'll likely reduce the perceived time spent waiting when multitasking (which is what computer users are getting more and more accustomed to doing -- especially on OS X). If multithreaded apps are able to run background threads on another processor, perceived response time will also be reduced within a single application.
Case in point: I have an ancient Dual Pentium Pro 200MHz machine running (I know, ack!) Windows XP, which is still very capable of running some heavy duty hardware design and synthesis tools --- though it takes substantially more time during builds than a newer machine, the fact that it's got an extra processor allows me to do other things while waiting for a build to complete, with minimal perceived lag time. Compare this with other single processor machines (at approx. 2 times the clock rate) that are totally frustrating to work on with the same software.
The conclusion? If you can afford a DP machine, and expect to be hanging on to it for a long time to come, get it! If, however, you have the funds and the inclination to upgrade to the next bestest fastest beast when it arrives, and you won't likely tax the current iteration, then the speed of a single G5 will do you just fine.