Sorry Keet for the misunderstanding about the OS.

For example Firefox does free up memory when you close tabs or a second instance, SH3D and/or Java apparently don't.

Firefox isn't a Java program. Java is great for many things including its memory management, but once a Java application allocates more memory to work, the way that memory is released once not not needed anymore is less predictable or may vary from a Java Virtual Machine implementation to an other. It's released without problem internally for other tasks in Sweet Home 3D, but even if the Java application doesn't need that internal free memory immediately, it won't be released for the OS during a long time.

have a switch to temporarily suppress 3DView updates while drawing in the 2D plan.

That switch already exists even if it's not so handy: hide the 3D view by moving the separator between the plan and the 3D view, close your home and reopen it. Then, the resources required for the 3D view will be requested only once you view the 3D view again.
But keep in mind that 3D resources are still needed to create top images of the furniture in the plan (unless you didn't select Top view option in preferences pane).

much does a graphics card help in the overall performance? One with faster or more memory?

Sorry, I don't know. When comparing the performances of the 3D view in the Java version and the JavaScript Online version on a given computer, I wonder if the bottleneck might be more in Java3D layer than in the graphics card.