I'm not sure what Apple currently do for 64/32 bit... they only seem to offer one set of install media. Do they just detect a G5 and install a 64 bit kernel and libraries if required? What about the rest of the OS, are the binaries there 64-bit or 32?
(The Pentium-M is quite an impressive bit of kit, and should help any worries that Mac users traditional complaints that PCs are hot and noisy. The 755 model gives over 1500 SPECint with a maximum power possible output of 33.5W).