OSX - 64 bit features for G5. Who knows for x86. Not that we can use the just announced 64-bit AMD dual cores anyway.
http://www.apple.com/macosx/features/64bit/
It seems like there's already a bit of a wait for the macbook, do you really want to add goodness knows how long for a product based on an unannounced chip from Intel?
Of course if you just want the fastest Mac out there right now, there's the Quad G5:
"The Quad G5 got the highest score we've ever seen on the CPU-stressing CineBench rendering test: 1,104. The Pentium EE840 overclocked to 3.6 GHz recently got a 667, and an Athlon 64 4800+ overclocked to 2.7 GHz scored 775. CineBench is a multithreaded app, so the more cores or threads your system can handle, the more efficiently your workload gets done." - from PCmag.com review
http://www.pcmag.com/article2/0,1759,1904636,00.asp