I have tested all this settings. In some cases you achieve a 2X CPU speed. But this depends a lot on your software setup... so you have to figure out by yourself.
I usually prefer fast single CPU (1.5 Ghz) than slow duals.
As usual, I agree with the Mactron and the Gary; after literally months of testing back in the day, true dual CPU access in OS 9 is unattainable, unless you were to use one of the few applications that supported it exclusively and used the machine for nothing else. In addition to that statement, there may be add-ons and other items that make a compatible application, suddenly unstable. As with Cubase, they gave it a valiant effort, but stability, hands down, is with leaving that shit turned off. Lastly, the reason Mactron and I opt for a really fast single over a really fast dual is that if you are not going to use the 2nd CPU and you have a dedicated OS 9 PPC, then why introduce extra heat into the equation. Alas, the MDD with an Xserve CPU (as mentioned about 100X) is the OS9 pinnacle in overall speed and stability as far as "Stock" Apple CPUs.
As a side note, Nuendo was the only Steinberg app (at the time) that was not originally written for a mac and was "ported" over, it is a complete re-write and nothing of Cubase was used. The OS 9 versions are useless and have many bugs and will crash very often. Cubase VST 5, on the other hand, was the last incarnation and the code was improved over the years... this Flagship app. is extremely stable, as each feature was added in a prior version, the next version would cleanup any small bugs. IMO, Cubase VST 5 is as good as it gets if you need Audio, MIDI, and Virtual Instrument Support. Other applications can beat it in MIDI, or Audio, but if you are working in the box with no outboard gear, overall CuBase or Logic (in OS 9) seem to be the best choice. That being said, back in the day, I only worked with Studio Vision when setting it up for clients, so I cannot comment since I never gave it a workout personally