@irishlamma: You might be a bit light on memory, but the key question is what's your video card?
C-IV is using 3D graphics, and needs the rendering to be offloaded to the graphics card while the CPU gets on with the game calculations. The minimum spec for C-IV graphics on PCs is 64 MB of video memory plus various built-in algorithms for handling 3D sprites - the graphics processor must be able to work out what's in front of what, where the shadows go etc ...
As Dojoboy says, it's been found that the game actually uses more than 64 MB of video memory, and the card will grab the extra from the system. But video memory is much faster (and more expensive) than system memory, so the more main RAM that's used the slower it gets. It sounds to me as if 128 MB video RAM is a good bet to ensure good graphics with no choppiness.
On a Mac, Dashboard uses Core Graphics to render the ripple effects when you add new widgets. The ripple doesn't happen unless unless you have a minimum spec card for Core Graphics. I would suggest looking at the video requirements for Core Graphics as a guideline for good C-IV performance.
If you have a combination of low system RAM *and* low video RAM then your system will be really struggling, as the video steals memory from the game, and the game has to page out to disk.
Extra system memory is pretty cheap these days, on Mac or PC, and your Mac will benefit all round from an upgrade. But a new video card for a Mac is a pain in the wallet, and only benefits games and high-end graphics applications.
Firaxis can be cavalier about telling PC owners to upgrade video because the PC video cards are cheap. Video card replacement is not usually possible for laptop owners, on either platform, but PC owners are more used to replacing their kit in months rather than years.