VirtualBox base memory
 

I am creating a Windows XP Pro virtual box to run on my Chakra Linux system (it is based on Arch). I only have 1.25 GB of physical memory on this laptop and so need to be conservative with the memory I allocate to the VM. So I was wondering if I set the base memory of the VM to something low, like 512 MB, if it will automatically use more memory if it is available.

The answer is
 

I have to say that I believe that it will use ONLY the memory you allocate.
That said, I have run XP in a virtual machine with 512 MB and it was fine. Win7 needs only a little more, but I run it in 944 MB when I can. (It runs faster that way.)
Your machine is a little memory constrained to run any recent version of Windows containers, but you might be fine with only one.