Hi Chris,
From the manual, all I can find is the information on this link: http://www.3cx.com/manual/3CXPhoneSystemManual5/phone-system15.html .
Unfortunately, this does not include the precise system specs that you require. I can tell you that I ran the server on a P3 1ghz machine with 512 mb of ram for months in a small 6 user environment and did not experience significant problems unless there were many simultaneous calls going on. In that case, users would experience lag and skipping. Since, the system has been upgraded to an AMD X2 5000+ with 2gb of ram and CPU usage is very low even with simultaneous calls going on. This system is just an e-machines pc that was purchased at a retail location, nothing special.
So, my only system specs come from my experience so I apologize. I invite other users to describe their servers and deployments so we can further gauge system spec requirements. And Chris, please let us know what you were planning on running for your 3cx application.