As I recall.....ArchiCAD 6.5 was not supported by OSX; You needed to upgrade to ArchiCAD 7 at the very least and preferrably to 8. That was reason I upgraded when I moved to OSX.
Graphisoft will give you an OSX compatible working copy of 6.5 as part of their 'legacy' software but I've never needed it so I never tried opening it.
I had a Blue G3 as well; After 6 yrs, it doesn't support OSX.4 - tiger. It will run ArchiCAD 8 or 9 with Panther (OSX.3) at the most.
Honestly I would go to 9 as soon as possible before version 10 arrives. I'm in the process of doing that as well since I'm not sure how much longer I can use 7 with the advances going on with Apple's Operating System. (On the other hand; 7 runs like blazes on a G5!!!! AND no crashes!)
HTH
Mark
Mac G5 - Dual 2.3 Ghz, 1.5 Gb Ram, OSX.4.2
ArchiCAD 7.__, Plotmaker 2.3 V2
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Mark R. Wallace AIA
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Mark Wallace Architect
Collegeville PA
Mark R. Wallace AIA
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MacBook 2.53 Ghz, Intel Core i5, 8 Gb,
Mac OSX (Sierra 10.12.6,
ArchiCAD 22 USA Full, +21, & 20.