julian, working on PC most of the time all my comments (and your assumption) from above are valid for this platform too, besides i don't see too much difference between a mac and a PC basically, particulary since apple moved to intel processors.
one could have long and abstract discussions about details like GUI, filesystems, thread handling and similar but for both neither twice as much RAM or harddisk capacity or GHz gives the user twice the power.
a significant amount of power is regulary eaten up by (what i call) *featuritis* of OS and apps, a certain amount lost by simple overhead immanent to the (hardware) system itself.
more impact on the performance has the design of a system and it's components. regarding media applications PPC simply suited better than the pentium 3/4/5 and the core duo/core 2 duo beats them both, SCSI performed better than IDE (pATA), sATA and SAS (serial attached SCSI) is more efficient, chipsets based on ICH (integrated controller hub, the successor of crossbars) give you more throughput than the southbridge/northbridge design, ect, ect, ...
remember the move from 16 to 32 bit computers in the mid nineties - a huge leap, besides the addressable RAM we got 32bit grafic (*true color*) and 32bit audio and the transition lasted ... a while.
what will be the advantage of 64 bit aside from addressable RAM? aside from x-ray pictures (fine greyscale) no graphical advantage, we don't expect audio to use 64bit in the near future, quality of video formats is actually decreasing (mpeg compressed and still 32bit), representation of unicode characters will still be 2 byte (16 bit), ect ...
so what should we expect overall: large overhead on every edge of a system because we will need 32bit compatibilty everywhere for a long time and it wouldn't take me wonder if we would see some disappointed faces around the table finally ...
christian