With so many new games benefiting from having 2GB or more of RAM (Battlefield 2 comes to mind), and the ever-increasing demand for more RAM to begin with, it's no surprise that OCZ is offering a "Vista-tailored" kit of a whopping 4GB, relying on dual 2048MB sticks. Available in both DDR2-667 and DDR2-800 speeds, the "OCZ Vista Upgrade" series probably doesn't do anything that other OCZ RAM can't, but having 4GB kits available is quite novel, at least for desktops.
The DDR2-800 kit is only available in 2GB currently, and can be seen here, the 4GB kit can be seen here. Odds are, as with XP, as people begin to upgrade, they will be more inclined to go with larger amounts of RAM to give themselves a nice buffer zone for future software.