An announcement made yesterday in the Journal Nature claims that researchers had a breakthrough and created a memory circuit with 100 billion bits per square centimeter, which is said to be about 100 times dense than current memory circuits.
"Researchers in California said they have created the world's densest memory circuit, one that's about 100 times denser than today's standard memory circuits, while remaining as small as a human white blood cell. Scientists from the California Institute of Technology and the University of California, Los Angeles, reported the development in Thursday's issue of the journal Nature. The circuit has 160,000 bits of capacity, compared with previous generations of molecular circuits that were demonstrated at 64 bits."
Don't expect 400GB of RAM to be commonplace anytime soon though. It will be more than a decade before the circuits reach mass production.