Search Engine Friend Dynamic Pages

Hey all, Just a quick question regarding developing search engine friendly pages. Basically I am re-writing my site so that say mysite.com? page=support&action=help becomes mysite.com/support/help/ Reason why is it looks nicer and is search engine friendly, as some search engines cannot spider URLS with query strings. Anyhow my preferable method of achiving this is having all pages raise a 404 error, which then loads my custom 404 page which shall output a "200 ok header" and load the appropiate content. I have heard somewhere though, that despite overwriting the 404 header, search engines wont index the page because they think it does not exist... is this the case? If so I shall use another method, such as one where I crop the *.php extension off my files and force apache to interpret them as mime type php. Thanks for your time everyone