I would be a bit disappointed if only 3GB of RAM is present. Advanced 3D graphics game with huge map can definately gain performance on 64-bit OS with lots of RAM. They can even preload adjcent maps to reduce loading time, so the "Loading..." screen would be barely visible (You'd still got to render it).

Last time I checked a single 8GB DDR3 module cost around HKD 270 from official dealer, if they can make 64-bit of OS in XBox ready on-time, it's pretty desirable to outrun other console to have 16GB installed on default configuration with slight increase in price.