You don't need to build an "nVidia kernel". Rather, you need to build the kernel module and the drivers, or download an RPM or something. Actually, you shouldn't need to do more than download and execute the new nVidia drivers at this point - supposedly their installer is pretty much fool-proof. The drivers can be found on their website.
As for RedHat 9 - if it's not too much of a hassle, I would recommend putting it on, as it'll save you from downloading the umpteen million security fixes that you need for RedHat 8.0. The only even relatively-tricky part of installing is partitioning, and the RedHat installer will prevent you from doing anything too horrible or wrong.