What game are you running that requires 12 GB of RAM? I'm all for advancing computers but I can't think of any game that would require that much at this time. I doubt games over the next 2 years would even approach that since vendors are developing for a "common" customer level of hardware.
As for finding a good gaming desktop, why not build one yourself that meets those requirements? The cost to find a pre-built system that matches those requirements would be much higher than making it yourself.