If you still have an 'old' style G5 and are lucky enough to be able to run 10.3.3 with all relevant updates - this is the most stable solution for V3.3 G21. However, If you have a 'newer' G5 which came with Tiger installed - you wont be able to downgrade back to 10.3.3 - so in this case - 10.3.9 with all relevant updates and V3.3 G21 is most stable. However, you will have to select 'slow disks' to get arround issues of catalyst crashing. (an open GL bug - i've been told....)

As for a stable platform to run v3.31 - in theory - the answer is 10.3.9 with all relevant updates and up to date ATI drivers etc. However, I havent had a chance to fully test this yet.... So I'll be sticking to 10.3.3 + V3.3 G21 until I can prove that v3.31 is show worthy and stable on 10.3.9. At the moment we're staying away from tiger...