Just upgraded XPS 8700 to Windows 10 and now just purchased an SSD. I have a few questions :D

So i just upgraded from Windows 8.1 that came pre-installed on my computer to windows 10.  I have read so many positive reviews of how an OS on an SSD can be mind boggling fast!  I have read that the serial number is already programmed into the motherboard so that i would not need it should i reinstall windows.  My question is do I need a windows 8.1 or windows 10 to install onto the new SSD?