On a 64-bit operating system, Autodesk Civil 3D is able to use whatever system memory is available (virtual memory and physical memory).
For 64-bit operating systems, optimal memory usage can be obtained by setting your virtual memory to twice the amount of physical memory you have. For example, if you have 4 GB of physical RAM, set your virtual memory to 8 GB. If you have a need for more memory, you will obtain better incremental performance if you add more physical memory rather than simply increasing the virtual memory over the 2:1 ratio. For example, increase your physical memory to 6 GB and then increase your virtual memory to 12 GB.