i have a g4 dual 1.25 i'm about to unload. my plugs are ALL RTAS. PT 7.x runs terribly on it. can't get nearly the plug counts i was getting on 6.4 with panther. tiger is a HOG and the 30% gain in RTAS "power" that digi added with ver 7 doesn't help at all. some sessions i ran with little trouble won't play AT ALL in ver 7. if you're running mostly TDM plugs (esp. the verbs, MB Comps, linear phase EQ's), you'll be fine.
on another note, the machines themselves are VERY noisy compared to a G5. i'm using a dual 2.3PPC machine right now and it is whisper quiet. my G4 is ten times louder than this box - without a doubt.
buying a quad-core Intel Mac, 2.66 this weekend. can't wait.