If I doubled my RAM, would my memory usage go down?

My PC currently only has 1GB of RAM, and I am currently using 86% of that. With 2GB of RAM, would I be using 43% right now?

Answer

Yes, your math is correct, to some extent.

However, note that modern operating systems use excess memory for caching. This article does a good job of explaining how OS memory management works (written for Windows Vista, but generally relevant for all operating systems); basically, it tries to use up your free (idle) memory for its cache, and when needed by another program, that memory is released again.

So the OS might appear to be taking up more memory, but it is only for caching purposes and can be freed at any time.