I held down the 6 and 4 keys and booted up into 64 bit kernel on my mac pro 3.1 fine. I checked and all my extensions are 64 bit, but not all software is. So far, all of my stuff works fine- the printer, scanner, trackpad, revolution mx mouse and web cam, as do all of the programs that I have tried to use. What is the benefit of running my mac in 64 bit kernel? Can I set something so that it will automatically boot up in 64 bit every time?