<P>I use Cross Linux from Scratch (CLFS) on my ix86 and x86_64 machines in a cluster of six machines.  I also use it on an old G4 PowerPC machine.  It is easy, if somewhat time consuming, to install.  The book provides instructions for a multilib x86_64 build.  It is one of the most standards compliant x86_64 distributions I've come across.  Switching between the 32-bit and 64-bit tool chain is as simple as using the -m32 or -m64 switch.  I've tried other multilib x86_64 distros that provide a 32-bit tool chain AND a 64-bit tool chain rather than one multilib tool chain like CLFS.  Keeping architecture specific configuration files (e.g.,mysql_config) separate is done by appending a -32 or -64 to the file name.  CLFS provides the multiarch_wrapper program.  This is a small C application that reads the USE_ARCH environment variable which is set to either 32 or 64, and selects the correct file to use.  This is the only wrapper program required.  32-bit applications run like Firefox don't require wrappers to use 32-bit plugins.  Just build the 32-bit version of Firefox and install the plugins just as you would on an ix86 platform.  Instructions for over 1000 packages can be found at the CBLFS Wiki (http://cblfs.cross-lfs.org/index.php/Main_Page).  There is also a CLFS Hints Wiki (http://hints.cross-lfs.org/index.php/Main_Page).  Boot time is acceptable.  One of my machines (http://cross-lfs.org/%7Earowland/bootchart.png) boots to run level 3 in under 25 seconds.</P>