You're going to need to have kernel-source installed (the version has to match your running kernel) and configured.
You're then going to need to do an rpmbuild --rebuild nforce-whatever.src.rpm to actually create a usable RPM.
Then, look through the output of rpmbuild to see where it writes the rpm to (usually somewhere under /usr/src/redhat/), and rpm -ivh that file.
However, I'm not sure 256 will even compile on RH9, due to the kernel changes. Go ahead and try it, but if you get errors about remap_page_range, then that's the bug you're seeing.