Hi, check out the utility that ought to have been installed (at least when you have the Mac Dev Tools installed) called gcc_select.
Here's a short snippet from it's man page:
GCC_SELECT(8) MacOS X GCC_SELECT(8)
gcc_select -- Switch the default system compiler
gcc_select [-n] [-force] [2 | 3 | 3.<number> | 4.<number> ]
[-h | --help] [-v | --version] [-l | --list] [-root]
o 2: Select gcc 2.95.2 as the default compiler.
o 3: Select gcc 3.1 as the default compiler.
o 3.x: Select gcc 3.x as the default compiler.
o 4.x: Select gcc 4.x as the default compiler.
s**lil*; $*=join'',sort split q**; s;.*;grr; &&s+(.(.)).+$2$1+; $; =
qq-$_-;s,.*,ahc,;$,.=chop for split q,,,reverse;print for($,,$;,$*,$/)