First of all, thank you very much for taking the time to post!
ClamAV indeed does not contain definitions for Mac OS 9 (or earlier) viruses, which would not affect a Mac OS X only installation. It does however contain definitions for viruses of other platforms (preventing a Mac user from passing a virus along onto a network or allowing him to detect potential outbreaks) as well as cross-platform viruses or malicious applications that, while not specifically targeted at the Mac, could affect it — think some Java applets, for example.
As Mac OS X is essentially a UNIX-like operating system, the ClamAV project has recently pledged improved support for it, which includes the adding of any potential Mac OS X malware to the definitions list. Of course, this implies that the community reports such programs to the ClamAV authors and that they in return judge the threat of significance so that they act upon it.
ClamXav provides a GUI on top of ClamAV and, as such, wouldn't provide more protection than mastering ClamAV from the command line (with the notable exception of the real-time scanning it now features).
If a Mac OS X virus or worm appears in the wild, it would need to be added by the ClamAV developers to the database. While there cannot be any guarantee that it will be (much like with any other anti-virus application) everything seems to indicate it will.