The number of Mac OSX viruses/trojans is tiny.
Six. [1]
Four are proof-of-concept and harmless. One only affects PPC Macs and relies on the user downloading an attachment from iChat to their desktop and then opening it. The last one relies on the "Safe file" setting in Safari and from what I can tell is a theoretical vulnerability, that appears to have no known actual exploit.
i.e. Since Mac OSX 10.0 was released in 2001, there have been two potentially harmful viruses, neither of which infected more than 49 computers in the entire world...
That's not to say that Macs are inherently virus free as they aren't. However, I'd guess that you are more likely to be struck by lightning...
If you want AV for a mac, then get www.clamxav.com/ as its free and ClamAV is a well respected AV engine.
Calling 10.5 a minor upgrade to 10.4 is a bit like called Windows XP a minor upgrade to Windows 2000. Whether it's worth 100 quid though is obviously debatable, but it's certainly much more than a service pack.
On Windows, if you run a good AV package like AVG or Avast and don't open unknown attachments willy-nilly then your chances of having a virus problem is fairly low too. it's all a matter of user education really.
Regards,
Rob...
[1] www.symantec.com/security_response/threatexplorer/azlisting.jsp?azid=O