What I do for openafs is this:

rpmbuild --rebuild openafs*src.rpm --define "build_userspace 1" --define "build_modules 0" 

which builds me all openafs*i386.rpm's
then 

rpmbuild --rebuild openafs*src.rpm --define "build_userspace 0" --define "build_modules 1" --define "kernel X.Y.Z" --target i686

and then --target athlon
which builds me modules for i686/athlon.

example
rpmbuild --rebuild openafs-1.2.11-15.5.SL.src.rpm --define "build_userspace 0" --define "build_modules 1" --define "kernel 2.4.21-15.0.3.EL" --target i686

rpmbuild --rebuild openafs-1.2.11-15.5.SL.src.rpm --define "build_userspace 0" --define "build_modules 1" --define "kernel 2.4.21-15.0.3.EL" --target athlon