How to use separate website ,,Page not found'' in correct way?

I would like to use in example:
RewriteRule ^widgets-([a-z]+)\.html$ http://www.example.com/new-widgets-$1.html [R=301,L]

1. How I do for all not founded pages on server separate page or I should define each time names of local pages?
2. Should I use (?f=78):
RewriteRule ^debating-room/$ http://www.example.com/forumdisplay.php?f=78 [R=301,L] or
RewriteRule ^widgets-([a-z]+)\.html$ http://www.example.com/new-widgets-$1.html [R=301,L]