Try rendering by starting Carrara, then go right to the render room and render the file with the batch option.  Don't load the file -- as if you were going to work on it.  I've been able to render much larger files by doing this.

Also, on a PC, I will monitor memory usage, and when memory usage gets to ~1.7 Gig, I pause the render, shut down Carrara, restart, and then start the render from the pause point.  This resets the memory usage, and I've been able to render files that consistently crashed when trying to render via the standard render mode.