If you are planning to use complex IVR scripts (DB access and custom scripts) consider to double the RAM.
If you need to store a lot of logs and call recordings, please evaluate a large HDD space. Calls recorded in wav format need about 1MB per minute, in GSM format about 300k per minute. CentOS default root partition is 50 GB, if you need more space set partition size manually during installation.
For security reasons, please consider to install a proper firewall in front of your XCALLY server, especially if it is deployed on the Cloud.