How to Customize 404 messages

I'm trying to add customized 404 (and other) error messages to my site. All of the help I found on the web, says create an .htaccess file, which I did, but i have also found info saying you need to have the AllowOveride set, is this done in the .htaccess file or in apache2.conf?

Here is what my .htaccess file contains:
ErrorDocument 404 /404err.php

Here is the section of the apache2.conf file that I think needs editing?:
# Customizable error responses come in three flavors:
# 1) plain text 2) local redirects 3) external redirects
#
# Some examples:
#ErrorDocument 500 "The server made a boo boo."
#ErrorDocument 404 /404.phpl
#ErrorDocument 404 "/cgi-bin/missing_handler.pl"
#ErrorDocument 402 http://www.example.com/subscription_info.html
#

#
# Putting this all together, we can internationalize error responses.
#
# We use Alias to redirect any /error/HTTP_<error>.html.var response to
# our collection of by-error message multi-language collections.  We use
# includes to substitute the appropriate text.
#
# You can modify the messages' appearance without changing any of the
# default HTTP_<error>.html.var files by adding the line:
#
#   Alias /error/include/ "/your/include/path/"
#
# which allows you to create your own set of files by starting with the
# /usr/share/apache2/error/include/ files and copying them to /your/include/path/,
# even on a per-VirtualHost basis.  The default include files will display
# your Apache version number and your ServerAdmin email address regardless
# of the setting of ServerSignature.
#
# The internationalized error documents require mod_alias, mod_include
# and mod_negotiation.  To activate them, uncomment the following 30 lines.

#    Alias /error/ "/usr/share/apache2/error/"
#
#    <Directory "/usr/share/apache2/error">
#        AllowOverride None
#        Options IncludesNoExec
#        AddOutputFilter Includes html
#        AddHandler type-map var
#        Order allow,deny
#        Allow from all
#        LanguagePriority en cs de es fr it nl sv pt-br ro
#        ForceLanguagePriority Prefer Fallback
#    </Directory>
#
#    ErrorDocument 400 /error/HTTP_BAD_REQUEST.html.var
#    ErrorDocument 401 /error/HTTP_UNAUTHORIZED.html.var
#    ErrorDocument 403 /error/HTTP_FORBIDDEN.html.var
#     ErrorDocument 404 /404.php
#    ErrorDocument 405 /error/HTTP_METHOD_NOT_ALLOWED.html.var
#    ErrorDocument 408 /error/HTTP_REQUEST_TIME_OUT.html.var
#    ErrorDocument 410 /error/HTTP_GONE.html.var
#    ErrorDocument 411 /error/HTTP_LENGTH_REQUIRED.html.var
#    ErrorDocument 412 /error/HTTP_PRECONDITION_FAILED.html.var
#    ErrorDocument 413 /error/HTTP_REQUEST_ENTITY_TOO_LARGE.html.var
#    ErrorDocument 414 /error/HTTP_REQUEST_URI_TOO_LARGE.html.var
#    ErrorDocument 415 /error/HTTP_UNSUPPORTED_MEDIA_TYPE.html.var
#    ErrorDocument 500 /error/HTTP_INTERNAL_SERVER_ERROR.html.var
#    ErrorDocument 501 /error/HTTP_NOT_IMPLEMENTED.html.var
#    ErrorDocument 502 /error/HTTP_BAD_GATEWAY.html.var
#    ErrorDocument 503 /error/HTTP_SERVICE_UNAVAILABLE.html.var
#    ErrorDocument 506 /error/HTTP_VARIANT_ALSO_VARIES.html.var

So I guess the question is, what do i edit, my .htaccess or my .conf?