Here are some more suggestions to reduce your memory usage.
1. Fastcgi-configuration
In /etc/apache2/mods-available/fastcgi_ispcp.conf change this
Code:
-idle-timeout 300
# -singleThreshold 100
to this
Code:
-idle-timeout 300 \
-singleThreshold 100
This causes that the the last fastcgi-processes for a website terminates when it is not used.
2. Apache-prefork
Change apache to use prefork:
Code:
apt-get install apache2-mpm-prefork
You can also modify apache2.conf:
Code:
<IfModule mpm_prefork_module>
StartServers 12
MinSpareServers 12
MaxSpareServers 24
MaxClients 100
MaxRequestsPerChild 0
</IfModule>