What are the minimum system requirements for FreePBX Distro 5.211.65-11

I currently have FreePBX Distro 5.211.65-11 configured and running on a server with a Xeon 4-core 2.4 GHz processor, 12 GB memory and 1 TB Harddrive. with about 60 phones and 20 concurrent calls. I have an older server with a Pentium 2-core 2 GHz processor, 4GB memory and only 2 160 GB hard drives. What is the mimimum that I should upgrade the older system to, to be able to use it as a emergency back up to the primary system. There is not much money in this years budget for a brand new server. I will push for a new server next budget.