ISPConfig itself does not need much resources, its just a set of php scripts and not a daemon, So it does not use any ram if you are not currently logged in or if the server script is run once a minute for a second or two. The resources are used by other software on your server like mysql and the email system. If you installed a full setup as described in the perfect setup guide, then this contains setup e.g. a full spam spam and antivirus scanning daemon etc. and such a full setup requires more then 256 MB RAM. The recommended RAM amount for a full setup is 1GB or more.
What you can do is that you disable some services like the spam scanner and also disable innodb in mysql to use less RAM.
Disable spam and antivirus scanning:
And to disable innodb in mysql, edit the file /etc/mysql/my.cnf and add the line:
in the [mysqld] section and then restart mysql.