There is an issue with AppleNForceATA and other ATA kexts in that they are written using 32bit addresses. This causes problems with using ~ 3gb - 3.5gb of memory. The fix listed in my signature comes from Slashack. This person modified the kext using 64bit addresses which solved the problem for me. In your case though if its not broken don't fix it.

OSX can run in 64bit mode when this is offered by the processor. It may however cause problems running PPC applications through rossetta. Some applications can take advantage of 64bit but most of the kexts (drivers) out their are not 64bit. OSX 10.6 "snow leopard" should be a big jump in the right direction. I believe we will see 64bit kexts across the board and hopefully more 64bit software support

Im not sure why you are having problems with enabling 64bit with the new kernel.

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