As a Quad G5 owner, might I recommend OS X "Sorbet" It's an unofficial merging of OS X 10.5 with PowerPC builds of 10.6 components. Even on ancient G3's it out-performs both 10.4 and 10.5 in benchmarks
Yup get ready for the Mac Pro that was on sale at full price until a few weeks ago at $7000 to be removed from the compatibility list for whatever comes after Sonoma.