Well my Dual G5 with only 1.5GB of RAM has still been rock-steady with both iDVD6 and the Quicktime updates.
Basically, don't upgrade your software if it's working, because then it won't work. Let others beta test the problems first.
Or make sure you have stand-by hard drive clones with the old software ready to un-upgrade. I always have a cloned backup (pre-upgrade) in case there is some glitch. This is really the single most important thing any computer user on any platform (Mac/Windows/Linux/OS2Warp...etc.) should do, but it far too uncommon. Things on Macs, as much as we want to believe they "just work" , sometimes they don't, and a clone is a giant "un-do" for your computer. Don't leave your system without one. Priceless.
buy a new Intel PC, I mean Mac, that will be obsolete by September when the new Intel chips come out that will require a new upgrade.
While a dual G5 should be far from obsolete, Macs have a great track record of NOT becoming obsolete. My 9600 with a G3 upgrade is running 10.3, and my G4 TiBook is still going strong, both very useful machines. Do I run iDVD6 on it? No, nor would I expect to, but iDVD3/4/5 run fine, albeit slow. Yes things are fastest on the MacBook Pro!
No doubt, iDVD needs to improve it's handling of audio and the spinning beachball issues. Just a warning about 12 bit audio would be huge.
I've used every iLife version, and they've worked great for me, save a G5 specific issue when iDVD4 came out. Doesn't mean you're not having real problems, but it also doesn't mean that all iDVD QT G5 users are having the same issues.