I'd like to drop some of the older 4.3 versions, so please stabilize sys-devel/gcc-4.3.6-r1. You will almost certainly run into runtime issues with packages linked against libstdc++ due to symbol versioning (see bug #297685). The error will look something like /usr/lib/gcc/x86_64-pc-linux-gnu/4.3.4/libstdc++.so.6: version `GLIBCXX_3.4.11' not found. You'll need to rebuild these packages with 4.3. The dependency chains between packages can sometimes be difficult to figure out so expect that it may take some effort.
amd64 ok, rebuilded @system without problems
amd64: archtester / # gcc-config -P x86_64-pc-linux-gnu-4.3.6 * Switching native-compiler to x86_64-pc-linux-gnu-4.3.6 ... emerged system fine.
amd64 stable
x86: ok ( rebuild @system )
ppc done
ppc64 done
x86 stable, thanks Zlogene
arm stable
alpha/ia64/s390/sh/sparc stable