How to change Notes editor to HTML
This might be a nice feature to add, but it is not something you can configure without rewriting code.  

It would actually take more than just setting the editor: notes display plain text, so if you were to give the box a WYSIWYG editor, you'd see a bunch of HTML tags in the content.  Building the feature would require adding the editor, modifying the display, and making sure that old notes display properly after the upgrade.

However, if you want to work around this, you can build your own equivalent of notes:


Create a new set of custom fields, called "Comments" or "Improved Notes" or whatever.  Have it used for Contacts, allow multiple records, and have the Tab display style.
Add an Alphanumeric / Text field called "Subject".  Make it searchable.
Add a Note / Rich Text Editor field called "Comment".  Make it required and searchable.
Add a Date field called "Date".  Make it required and searchable.
Add a Contact Reference field called "Author".  Make it required and searchable.
Add a File field called "Attachment".  Add additional ones if needed.
Now, go to Administer - Customize Data and Screens - Display Preferences, and uncheck Notes in the Viewing Contact and Contact Search sections.


You should now have a tab for your new custom notes, and people won't see the standard notes (so you don't have to worry about people setting notes in the wrong place).  A couple of caveats:


You'll need to transfer any notes from the standard system.
The "privacy" feature isn't available--either you can view all notes or none.
The date and author will need to be set manually, though on the other hand, you now have the ability to backdate notes or write them on behalf of others.