The feature is available right in the location bar of Opera 9.5. Hits begin to appear with the first key that is entered. This is in my opinion a great way of finding websites that you have visited but can’t remember the url of because it is sufficient to enter any term that you can remember that was mentioned on the website as long as it was textual.

The data that is needed is stored in the Opera profile folder and deleted once you clear the cache and addresses in the preferences. The Opera Preference Editor contains a setting called Max Visited Pages Index Size which defines the maximum size of the pages that are stored in the profile directory.

The default value is unlimited (-1). You can change the value to 0 if you want to disable that feature or to another value if you want to limit the size on the hard drive. I think it’s Kilobytes and not Megabytes that you have to enter there just like in the Cache setting.