In your proposed example you will run the Windows 7 OS and use it at least for running the Windows XP os. All by itself, that guarantees most of your 8 GB of RAM will be occupied before you open any applications.

Add to this that by design Windows will try to preserve as much RAM as it can and you’ll find that 8 GB will be not quite what you’re hoping for. 16 GB will do all the above plus leave elbow room for instances of many widely used applications.