If you want to try out Windows Vista in a guest, it will refuse to install if it is given less than 512 MB RAM, so you'll need that for the guest alone, plus the memory your operating system normally needs. So, if you want to run Windows XP on Windows XP, you probably won't enjoy the experience much with less than 1 GB of RAM. Basically, you will need whatever your host operating system needs to run comfortably, plus the amount that the guest operating system needs. Depending on what guest operating systems you want to run, you will need at least 512 MB of RAM (but probably more, and the more the better). Any recent Intel or AMD processor should do. You can install and run as many virtual machines as you like - the only practical limits are disk space and memory. So, for example, you can run Windows and Linux on your Mac, run Windows Server 2008 on your Linux server, run Linux on your Windows PC, and so on, all alongside your existing applications. Secondly, it extends the capabilities of your existing computer so that it can run multiple operating systems (inside multiple virtual machines) at the same time.

What does that mean? For one thing, it installs on your existing Intel or AMD-based computers, whether they are running Windows, Mac, Linux or Solaris operating systems. VirtualBox is a cross-platform virtualization application. How to Install Windows 10 as Virtual Machine in VirtualBox

Windows 10 virtualbox configuration