The RAM usage should not be that high.
Lets see if some corrupt Preferences are causing this, please create new Preferences as explained below:
1. Quit QuarkXPress if running.
2. Navigate to C:\Users\<Logged In User>\AppData\Local\Quark. Rename (or Delete) the folder named "QuarkXPress 9".
3. Relaunch QuarkXPress.
Are you able to open multiple files now?