Hey man. Yes I guarentee another G will be all you need. Just look at Soneji. He had the greatest all around computer going into civ4 but lacked in ram, only had one gig. This made all the differnce as you can see.
Truly, the OS takes a bit, your BIOS takes a bit for your internal intergrated graphics chip. Theirs a whole lot of this type
...< Snip >... that leaves far less for civving.
Others are wrong when they say 1gig is enough for everyone . They don't have the patience for huge maps like you so will never see large stress loads when dealing with BTS late game.
If you like mods they can add even more stress on the games ram limits. Playin custum 'extra huge' and scrolling over an enemy with 1000 units, it usually means tragedy without a prior save put down.
You can avoid this for low cost so I advise you go out a gets some. Ive seen mouses that cost more then two g sticks. Can't go wrong like in some CPU's cases