I'm running several wordpress sites and apache/suPHP is sucking the life out of my server.
Suphp is the slowest possible option. It should not be used for any cms systems like wordpress, joomla etc. Go to website settings and enable the suxec checkbox, then select php mode php-fcgi, the result will be a php setup that runs your site under the website user like suphp, just several times faster.
Then login on the shell and run thiese commands:
apt-get install php5-xcache
your server load will be much lower now and your sites will load fast.
is there a relatively painless way to switch an existing ISPConfig 188.8.131.52 installation with Apache2 to Nginx + php-fpm.
There isnt any and if you use any wordpress plugins that use custom rules in a .htaccess file, then its likely that you break the sites by switching to nginx. As I explained above, the switch is normally not nescessary, you just used bad settings for apache.