As of current version, you can change the email subject and some body text:
Go to folder
FileVista\App_GlobalResources\OriginalFiles\Languages
and copy FileVista-en.xml file (or FileVista-fr.xml for French) and then go to folder
FileVista\App_GlobalResources\FileVista\Languages
and paste the file here.
Now you can override this language file by editing xml with a text editor like notepad.
FileVista will detect this new language file automatically.
The lines you are interested are as follows:
<String key="666">{0} Event Notification - {1}</String>
<String key="667">An event has occured in a folder that you ({0}) were set to receive notifications from. The event details are listed below.</String>