I would suggest you look here:
http://osx.topicdesk.com/2005/11/04/new-buildsmart-release-2/
Dale Walsh (who did one of the articles for installing this SW for 10.3 some time ago) has made things tremendously easy to have an up to date Amavis and ClamAV installation.
Get BuildSmart, follow the included instructions, and it works beautifully, leaving the Apple installation completely untouched (stops the Apple-bundled versions, and then disables the launchd items for the same)
I’ve done this without issue on two 10.4.3 servers, one of which has a gets alot of use in terms of mail-services.
You’ll have amavis-stats and can see what it’s doing for you…
Also have a look at Alex’s items at that site, plenty useful !