For tech forums, I'd write enough about your problem so that it can stand by itself. After that, I'm fine with linking to a past thread if you feel it's relevant.
For community forums, I think bringing topics "back from the dead" will vary on a case-by-case basis. For example, if it's a generic thread on a presidential candidate, just start a new thread. On the other hand, if it's a very specific topic and there's relevant information in the old thread, feel free to bring it back. Just my two cents.
As for searching, we've done some modification to the vBulletin code to make our search a little more useful, but it's far from perfect. If you click on the Search link at the top-right of any page (just under the Log Out option), you'll find that there's also a choice to search using Google. Google's algorithms are far beyond anything we could offer, but has the disadvantage of only searching those pages that Google has indexed. This means that recent posts may not be included in Google's search.
Hope that helps, and thanks for joining us!