Actually, when I've tried to activate the Large Address Aware tag in the past, it's told me that it already uses more than 2gig, so afaik it's a 64bit app.
@OP - I've got 4g DDR3 and Win 7. Try turning harbour details down in the in-game menu, or particle effects in the same place. Failing that, use D3D antilag to improve your overall framerate.
