Are you trying to upgrade iTunes to 10.7 or 11.0 in order to sync a new iPhone? And you've found that your OS X 10.5 won't support the new iTunes? If yes, you need to buy OS X 10.6, it's not available via Software Update.
OS X 10.6 is available from the online Apple store, in the US by calling 800-MY-APPLE, or online here. After OS X 10.6 installation, run Software Update until there are no more updates available, at which point you'll be at OS X 10.6.8. At that point you can install the current iTunes.