I still use an old PowerPC G4 PowerBook at home both because I like it and because I hate the hassle of switching to an Intel-based Mac (have done that at work). The phone software update offer (to iOS4) in iTunes would only take me to a notice that I already had the latest version of *iTunes*. Fortunately, I called to ask some questions. [Edited to add: UNfortunately I called . . . ]

Was told definitely not to try a Mac-powered update to iOS4 except in the most recent iTunes with a recent Mac OS on an Intel-based Mac or that much would be messed up. (No mention of what would work on PCs but I assume there are some operating system requirements there, as well.) No syncing possible in the old system for iOS4, either. [Edited to add: This turned out to be incorrect! Thanks to posts below from nbs2 and pdxer, I discovered that the problem was in iTunes so I reinstalled iTunes and updated the 3Gs to iOS4 in 20 minutes. All works fine. Syncs fine. I'm leaving the original info here with the refutation in case anyone else gets bum advice from Apple Care.

So, I am finally forced to get a new laptop--because of my 3Gs, that was heretofore keeping me from needing a new laptop. Edited to add: No Intel-based machine needed. Apple just needs to can a cust svc phone rep.