If like me you thought G5 support was dropped to soon in the Logic product line you may be happy to hear that what this means is that Apple won’t answer your questions. I’ve been running Logic 9 on my 4 year old quad G5 for about a week now. Ran some rather heavy projects and there were no issues. Though I did have to bring Logic Node into the mix once.
My perception is that 9 is actually a little bit faster than 8 and possibly more stable. I don’t have any numbers to support this but it did feel more responsive.
The installation was easy. No hacks. Just pop in your disks and continue feeding for an hour or two, but this is a quad G5 with 12 GB of RAM. Not sure how much memory is required, and not sure if it works on a Dual G5. If you must upgrade it might be worth trying, worst case you have to get new hardware. It’ll happen eventually anyway.
One note is that I have yet to update past the version included on the installation disks, so I’m not sure if newer updates remove binary support for G5 or not.