Apple's site is sometimes hard to tell what version is the latest. I do the "check updates" and assume I have the latest and that Leopard will be the next one out but wanted to verify that.
'Update's are available via the 'Apple, 'Software Update...' menu item, or via 'System Preferences' ('/Applications/' folder) 'Software Update' utility.
You have 'Tiger' (MacOS X 10.4.x) installed. All related 'Tiger' updates are free - via the above menu item or ('System Preferences') application; or, by going to 'Apple Downloads' and downloading the respective 'installer(s)'.
'Leopard' (MacOS X 10.5.0) will not be an update - it will be a new purchase, self installation.