If you have to take a site down for some type of unavoidable maintenance task (and it's not a big enough site that you have a backup server), what HTTP status code should you have your server return to minimize the possibility that search engines will think the site is gone?

I found [this list of status codes](http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html) from W3C, of which the following seem applicable:

* 503 Service Unavailable
* 500 Internal Server Error
* 408 Timeout
* 404 Not Found

I think 503 is the most appropriate, but I don't know what search engines might prefer.