What are the minimum system requirements for Kali Linux? 1GB of RAM (2GB Recommend), 20GB of free space.

How much RAM should I allocate to Kali Linux?

1 GB of RAM and 20 GB of hard disk is the minimum requirements for kali linux. But if you want to run it smoothly get atleast 2 GB of RAM. I run it on 8GB RAM that too dual booted with Arch Linux.

How much RAM is enough for VirtualBox?

A minimum 4GB ram is required for Virtualization, anything less will make the process too alow or unusable. 8GB is recommended for running a single Virtual machine at a time. You will need more than 8GB if you are trying to run more than a single virtual machine at the same time.