Mostly 32-bit possible?

["Gary Hennigan" <glhenni@sandia.gov>]

I'm considering the purchase of a dual-opteron rig and have some
questions. In general I don't care about anything being 64-bit, except
my own applications. These are scientific applications developed using
MPICH (g++-3.3 + MPICH MPI libraries). Is it feasible/possible to just
install the 32-bit "testing" release, for example, and still be able
to compile and run my own code in 64-bit mode?

My main concern with going full-blown 64 is some commercial apps that
I rely on that currently don't have 64-bit builds.

TIA,
Gary