Wow this topic is right up my alley.
Yes the PPC is designed to be a 64-bit machine, but it really isnt. The architecture allows that headroom though, unlike the crapola x86, where the address lines are reserved. Note that IBM ships the 64-bit version if the ISA in the Power line(which WILL run PPC code minus Altivec). This architecture is still bar none the fastest for the desktop. It runs with a 7 CYCLE pipeline! It takes an instruction 7 cycles to complete, versus 22 for the P4, and 9 for the Athlon. That is why Mac-faithful can get away with unoptimized code and underclocked chips and still not be embarassed.
x86, I have explained my woes of this processor in another post. It is probably the SINGLE worst example of microprocessor design available today. Originally introduced in 1979 as the next generation 8-bit architecture, its longevity can only be attributed to that same stinking vat that M$ has come from, more programs are written for it. And so, more engineers are employed to eek more performance out this undisposed garbage. Imagine if this many engineers actually worked on a architecture with headroom, like the IBM 360. 1Ghz would have been achieved long long ago.
And why the hell would anyone other than the sci-comp guys need 64-bit computing? To address a terabyte of RAM? Besides, the common folk couldnt hope to afford a memory subsystem to saturate a processor like that, and winblows wont run any better with 64 bits. (but your local weather forecaster may benefit).
Alas, as the morons who bought windows have spoken with their spending dollars, x86 will live on. While computing's best hope has been quietly killed off. RIP Alpha, we hardle knew ye.
one more note to choa, though the SPARC may be reliable because of Solaris' excellent programmers, that rotating regfile will forever relegate it to the ass end of computing. It may break 2Ghz next century though on a .00001 micron process grown by microbes.