The 64-bit CPU and OS guys and the RAM guys seem to have been waiting for each other to make the first mass marketing moves. Now WinChip has made the first move with a 4-slot 16GB DDR2 kit.
I rather think this will actually get the ball rolling for 64-bit Operating Systems sales, up to now it hasn't been worth losing backwards compatibility with 16-bit applications for the small RAM and performance improvements of 64-bit.
(At the non-professional/enthusiast price point on readily available mainboards that support 4GB DRAM DIMMS, that is)
Winchip DDR2 667 MHz 16 GB Quad Kit