Memory wasn't a problem, it was processor power that was the killer for ClamAV - only used it for HTTP traffic.

My backup ADSL connection also has my mail server on it (static IP) and Astaro handles all that - Spam and AV scanning of incoming mail, and it does it extremely well.

I did think about dual WAN, but rather keep it all separately. Fingers crossed, touch wood etc. the FTTH has only gone done once for any length of time (~90 minutes) in the last 12 months since it was installed.