You can use more "memory" than you have physically installed, it's called virtual memory. When the operating system needs more memory than physically available it swaps physical memory data to the hard drive to free it up. That's all fine and good except it is slow. Real slow... And less reliable.
I know all that and, well, it is all fine and good. It runs fast enough to play my flight sims. And it's as reliable as my other 'puters. It's not real slow unless your comparing it to screaming gamer machines with broadband.