I have heard press about solid state holographic memory storage systems using three dimensions (rather than two in current address busses). This sounds a bit futuristic, though
Oh, and also I forgot about QDR-SDRAM. We have DDR-SDRAM (double-data-rate) but soon we will probably see quad-data-rate RAM using the same quad-pumped technology as the 4x100MHz Pentium 4 bus. I hope. Maybe.