Private email from Boris Herman:
Frankly, I don't know, nor do I have a G5 Mac to test. Apple is very vague about it, mostly marketing talk, even the latest "tech note" on G5 optimization at http://developer.apple.com/performance/g5optimization.html. Also, Apple's claims that Darwin is 64-bit are non-convincing to me.
Thus, this is an interesting question to post on both the gcc mailing list and on Apple's darwin-development list. Generally, I think that GPC can address 8 GB if the gcc back-end can do so -- correct me Frank/Waldek if I am wrong.
Regards,
Adriaan van Os