Related to my previous question:
Preventing Memory issues when handling large amounts of text
Is
You can try GC.GetTotalMemory:
Retrieves the number of bytes currently thought to be allocated. A parameter indicates whether this method can wait a short interval before returning, to allow the system to collect garbage and finalize objects.
The important thing to note is this part: "Retrieves the number of bytes currently thought to be allocated". This means that this method may not be 100% accurate - as long as you know this going in, you should be able to get a rough idea of your virtual memory utilization at a given point in your application execution.
Edit: Let me now offer a different solution that will probably be more productive: use perfmon and the CLR performance counters.