Agreed re the 4Gb limit - it was built with an older Core Architecture in mind, and will not utilise above 4Gb. Having said that you *might* gain some speed by using RAMDiscs ..... but don't hold your breath, its a lot of messing around for minimal gain.
Only try the latter if you are buying 12Gb for other reasons, don't buy that 12Gb just for Civ 4 - RAMDiscs or no RAMdiscs.