Lars,
I have one of the latest variety Powerbook G4 1.67s (1.5 GB Ram, 100mb HD, 128 MB Video Ram). These machines are definitely fast enough for Eclipse and I run it pretty much every day, along with the MyEclipseIDE add-on. It is fast enough to use as a main development system. That said, I also have WinXP desktop (Athlon 64 3200+, 1GB, 10K Raptor, etc..). The PC completely (not surprisingly) smokes the Mac laptop in regards to Eclipse speed. That said, I still use the Mac *much* more than the PC, due to OS X.. I would Imagine that a Dual G5 would be in the same ballpark as the PC, if not faster.
Hope that helps!
Ed