The problem could also be textures, Sins has this problem from day one, and it still is a problem, and thus one of the modders started a optimization mod. To reduce the texture sizes and so on, In Elemental, I get lag in late game, but I never had a problem in regards to out of memory (on Win7 64bit).
However, I would have thought that Stardock should release a 64bit (beta) version of the game before retail.