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, from S.L. 4.2

rpmbuild --rebuild openafs-1.4.0-8.SL.src.rpm --define "build_userspace 0" --define "build_modules 1" --define "kernel 2.6.9-34.EL" --target i686

rpmbuild --rebuild openafs-1.4.0-8.SL.src.rpm --define "build_userspace 0" --define "build_modules 1" --define "kernel 2.6.9-34.ELsmp" --target i686

rpmbuild --rebuild openafs-1.4.0-8.SL.src.rpm --define "build_userspace 0" --define "build_modules 1" --define "kernel 2.6.9-34.EL" --target x86_64

rpmbuild --rebuild openafs-1.4.0-8.SL.src.rpm --define "build_userspace 0" --define "build_modules 1" --define "kernel 2.6.9-34.ELsmp" --target x86_64