>gcc-4.0 -arch ppc -arch i386 -fno-strict-aliasing ...
>
>followed, unsurprisingly, by:
>gcc-4.0: installation problem, cannot exec
>'i686-apple-darwin8-gcc-4.0.0': No such file or directory

From at least OS X 10.4 on, Xcode installs both variants of compilers so 
it is possible to build both Intel and PPC archs on any system.  I 
regularly build complete fat Python installers on a PPC G3 running 10.4.

$ uname -p
powerpc
$ i686-apple-darwin8-gcc-4.0.1 
i686-apple-darwin8-gcc-4.0.1: no input files

Was this possibly on an older version of OS X, say 10.3?  Otherwise, it 
sounds like that system did not have a complete installation of Xcode 
somehow.