Mac OSX Leopard is not fully 64 bit but partially. It can address up to 32GB of RAM at the moment with no problem and run 64 bit apps. The kernel is still 32 bit though.
http://www.appleinsider.com/print/08...d_64_bits.html
Personally, Im anxiously waiting for Snow Leopard to come out which will be fully 64 bit. Snow Leopard will be my weapon of choice.
