A relatively easy way to get RPMs optimized for your architecture is to download the source RPMs for the packages you want to install/upgrade. Look for RPMs with 'src' in their name. Check out the RPM Howto
for details; you can use the following command, assuming you have a working compiler and any required libraries installed:
rpm --rebuild --target athlon someprogram.src.rpm
(assuming you have an athlon; I think the option is '--arch i586' for a generic Pentium build). Also, read the rpm and rpmbuild manual pages for more info. Many programs that you can get the source code for also have a 'configure' option similar to this.
I usually do this with anything I install. I don't know how much difference it makes, though; the performance improvement might be negligible.