Manage 404 Error Page

Hello,
I created my custom error page and it sends an email with
serverVariable("URL") to me, so I know what files users may request
which are not found. All of a sudden, I started receiving a large
number of emails. My questions are as follows now:

1) what's the best way to log errors: database or flat text file (in
terms of speed and use)?
2) what useful info can I collect on error page 404 as you may suggest
(serverVariable("URL") serverVariable("REMOTE_ADDRESS") are not just
enough)

Thanks