I'd suggest you to remove the old Java first. I would also recommend you to get the binary file instead of the rpm. It works great and I found it easier to install. I'm not a real fan of rpm's. The instructions are on Sun's homepage and are very easy to follow.
Now, the good question is: Do you need Java 1.5? I've seen many peoples upgrading packages for no apparent reason. If Java 1.4.x is working fine for you and you are not a developer in need of the latest features of Java, keep 1.4.x. If it ain't broken, don't fix it
Good luck!