All the G5s run 10.4, but if you're looking for a Mac OS 9 system to run Mac OS 9 software, I wouldn't bother, compared to just putting a single really fast CPU into a Digital Audio or QuickSilver, or (if you can find such an ugprade) an MDD that'll boot 9. I'd really only "recommend" a G5 if you're completely strapped for space and you need something that can run tenfourfox(1) and old Mac applications.
The G5s are kind of a neat historic footnote at this point. Incrementally, and considering only raw performance factors, ignoring the fact that most G5s were less expandable than the G4s, they made sense as the next generation of Power Macintosh. Seen as a whole product, they had fewer slots than the MDD and QuickSilver and held fewer disks than anything since the desktop G3, and they used a whole lot of power compared to their predecessors. In addition, they weren't universally faster than the G4s they were replacing.
In addition to all of that, as uniserver said, the Netburst-based Developer Transition Kit (which had like a 3.2GHz P4 with HyperThreading in it, so not even the fastest Pentium 4 or Netburst chip) totally killed it benchmarking (and doing actual work) against everything except the quad.
The Quad itself is a really interesting kind of sidenote/footnote for Apple. They were announced and sold after the Intel transition was, for sort of the same reason the "For Mac OS 9" Power Macintosh G4 was introduced in 2003. Apple needed something to keep Photoshop and Final Cut users happy between the 2.7GHz G5 and the impending release of the Mac Pro.
It's worth noting that the Quad isn't typical PowerPC performance. It is over twice as fast as every other PowerPC Mac ever built, including the 2.7GHz system, due to IPC improvements. That alone makes it really interesting but sort of unfair to talk about in some ways.
One day I'll probably pick up a G5 of some sort as they continue to drop in price. If I had to say I had an ideal one, it'd be the 2.3GHz PCI Express (dual core, one chip) model with air cooling, just because it's fast enough to do anything I'd ever do with a G5 (10.5, Office 2008, Creative Suite 3, all from when I had my Core2 iMac in 2007-2009) and one of the more physically reliable systems.
(1) Typical warning about how I feel old releases of Mac OS X should be kept off of the Internet because of potential security issues.
The G5s are kind of a neat historic footnote at this point. Incrementally, and considering only raw performance factors, ignoring the fact that most G5s were less expandable than the G4s, they made sense as the next generation of Power Macintosh. Seen as a whole product, they had fewer slots than the MDD and QuickSilver and held fewer disks than anything since the desktop G3, and they used a whole lot of power compared to their predecessors. In addition, they weren't universally faster than the G4s they were replacing.
In addition to all of that, as uniserver said, the Netburst-based Developer Transition Kit (which had like a 3.2GHz P4 with HyperThreading in it, so not even the fastest Pentium 4 or Netburst chip) totally killed it benchmarking (and doing actual work) against everything except the quad.
The Quad itself is a really interesting kind of sidenote/footnote for Apple. They were announced and sold after the Intel transition was, for sort of the same reason the "For Mac OS 9" Power Macintosh G4 was introduced in 2003. Apple needed something to keep Photoshop and Final Cut users happy between the 2.7GHz G5 and the impending release of the Mac Pro.
It's worth noting that the Quad isn't typical PowerPC performance. It is over twice as fast as every other PowerPC Mac ever built, including the 2.7GHz system, due to IPC improvements. That alone makes it really interesting but sort of unfair to talk about in some ways.
One day I'll probably pick up a G5 of some sort as they continue to drop in price. If I had to say I had an ideal one, it'd be the 2.3GHz PCI Express (dual core, one chip) model with air cooling, just because it's fast enough to do anything I'd ever do with a G5 (10.5, Office 2008, Creative Suite 3, all from when I had my Core2 iMac in 2007-2009) and one of the more physically reliable systems.
(1) Typical warning about how I feel old releases of Mac OS X should be kept off of the Internet because of potential security issues.