If you are planning to upgrade to Archicad 10, I would recommend a new Intel machine. The G5s are nice machines, but are now outdated. You also get the added ability to run Windows on it if needed (via BootCamp or Parallels, after you acquire a Windows license for the machine).
I'm not sure AC8 would run on it, though. I know for sure the installers for Archicad 8 and 9 do not run on the Intel machines, even though the programs themselves do.
Archicad is still cross-platform. Your installation CD will have both the Mac and Windows versions.
Most of the issues I have seen are simple pathing issues, where different file structures mean you have to re-link file locations between the two platforms.
Tom Waltz