How can I make a page:

http://www.site.com/error404.php

a custom 404 page for the entire server, not just htaccess for one site.

Peace,

EDIT: Answer is to edit httpd.conf with the Errordocument setting i need.

well I am not sure if you solved the problem but :
http://httpd.apache.org/docs/2.0/mod/core.html
has some info on how to do that

add the errodocument line:

ErrorDocument 404 /path/to/the/file/error404.php