I think Apple might have something up its sleeve but not G6.
Reasoning behind this is as follows. There has been a clamour for a while now about PCI-Express for GFX on the logic board. It would seem to me, that if Apple was planning on having the current revision around for a reasonable amount of time, that they would have spent the time reworking the logic board for PCI-Express based graphics cards.
The fact that they havn't, suggests to me that it isn't worth the effort because this logic board is soon going to be replaced by one that supports something new (read multi-core G5's) in the pretty near future.
Am probably miles off but it is something to think about.
Simon