404 for a page with bad arguments

[qoregexp]

I have a page that uses the arguments to determine what the content should be. Unfortunately, if a search engine requests the page with an invalid argument they never get a 404 because the page does exist. I am thinking of checking the arguments and sending a 404 if they're incorrect BUT I wonder what effect this will have on the search engines' listings of the page with valid arguments. I.e. right now google lists both mypage.php?arg=good and mypage.php?arg=bad. If I send a 404 when their bot requests the bad arg version, eventually they will drop it (I hope) but will they also drop mypage.php?arg=good? I don't want that to happen. Does anyone know how this works? Would it be different for different search engines?

[Burf.com]

You could write all the pages out to a phsyical file as search engines prefer this

What about redirecting it to the home page for invalid links? Maybe that would cause dup problems?