First off, let's just assume you want to do this on your old single processor G4 tower, even though it's probably a good time to get a newer, more powerful Mac. If you insert the install disc, odds are that OS X "Tiger" will see your G4 as an older, unsupported model. The workaround is to remove your hard drive, insert it into a newer Mac, then install the new system on the drive. Once that's done, remove it and put it back in your old G4 and you should probably be okay (until Apple starts shipping "Leopard"). All of this is based on your having a newer Mac laying around or a good friend who trusts you enough to let you shove hardware into his machine, then safely remove it.