The page at http://trac.macports.org/ticket/27925 describes how to install the compilers with 32 and 64 bit (aka universal) support.
Add "universal_archs x86_64 i386" to macports.conf and update the installed ports with "sudo port upgrade --enforce-variants gcc43 +universal" and it will re-install the compiler and build the i386 libraries in additional the the x64 libraries.