Using 64bit Windows, how much RAM is sensible for running VisualStudio?

Visual Studio is only 32-bit. So anything above 3GB wont help a single instance. Running more instances should behave better (if memory usage is high per instance).

I've used two machines, on with 4GB and one with 8GB - Couldn't tell the difference between them and I was doing things pretty similar to you. If 8GB is not that much more - it could be worth investing now otherwise 4GB should be plenty.

More then 4GB only makes sense if you are planning to run 2 or more Virtual Machines and Visual Studio at the same time.