Autodesk released a Performance Monitor for AutoCAD, including AutoCAD Civil 3d over at the Labs site.
The Performance Monitor is a Windows desktop gadget that provides a color-coded status bar indicating the level of usage of virtual memory by AutoCAD products. When large amounts of virtual memory are in use, the running process may become unstable.
This should help you visualize your drawings memory usage and possible impending doom! It includes some nice status information and some system variable to help you tweak your drawings performance.