The new page looks good, almost like one of those ppc landing pages for 404s.

Its a nice 404 page, but I would not send other types of traffic to them as those other errors are for different reasons:
400 - It almost always means bad programming of the client system and/or your Web server.
401 - Generally this error message means you need to log on (enter a valid user ID and password) somewhere first
403 - This indicates a fundamental access problem, the most common reason for this error is that directory browsing is forbidden
500 - Internal server error, it is not a client-side problem and can only be resolved by fixes to the Web server software

asides which, you cannot use a full URL in a 401 ErrorDocument directive, you need to use something on your server like /error.html