It really depends on the thread/post count of your forums and the server you're hosting them on. 800 is fine for a smaller site as the result set will be less - on a 1+ million post forum you'll like create a large/long query using 800 results which may slow your site down or make searching a laborious task for you and your members.
For larger sites (1 million posts +) these is an
enhanced search add-on - but it's only really applicable if you have a dedicated server.
Cheers,
Shaun