I will also say this. If you have any custom links to recent threads, make sure that you update these to the new format for 6.4. If you do not, then when this is hit it will do a full table scan on your posts table whenever that link is hit. This will essentialy cripple your forums.
Basically, hover your mouse over the "New posts in past 24 hours" link and look at the variables that are passed to the dosearch.php script. You'll need to make sure that you update any custom links to the search engine with these variables.
