HT1443: What is an OS X reference release?Learn about What is an OS X reference release?
What version of iTunes?
Confirm the OS you're using? Your profile says 10.4.11 Tiger; this is the 10.6 Snow Leopard Forum.
If you are indeed on Tiger, you won't be able to use the latest iTunes version. The specs say OS X 10.5.8 or later.
You're not getting any update notices from Software Update because your Tiger system is fully up to date.
That OS change is not an update. It's a paid upgrade, and Leopard install discs are no longer sold by Apple. On Amazon MP they start at around $140.
If your Mac meets the specs, you can go to Snow Leopard for only $30. That is available only from the online Apple Store now. It's no longer available from High St stores.
However, that may bring with it another set of problems with software updates for third-party applications, some of them not free.