The latest version of XCode allows you to specify separate settings for x86 & PPC code, although it isn’t really obvious how to do it. After some digging I figured it out.
Some of the build settings such as SDKROOT & GCC_VERSION allow you to specify which architecture they apply to by appending _ppc or _i386 to the settings name. Here’s how I’m able to build a universal binary that will run on OS X 10.2 or later (which requires it to be built with GCC 3.3) as well as x86.
First, set up the project & target settings as required for building the x86 version. Then add the following variables to the build panel of the project or target settings window (hit the ‘+’ button to add a variable & edit the name).
MACOSX_DEPLOYMENT_TARGET_ppc 10.2 GCC_VERSION_ppc 3.3 SDKROOT_ppc /Developer/SDKs/MacOSX10.2.8.sdk
Those settings will override the defaults when building a PPC binary, while the defaults will be used when building x86.