I'm not sure if this is correct, but I'm pretty sure all of your pages will need to be php pages if the registration is going to really do much. You are going to need some php code on each page so the page can tell whether a user is registered or not.
I currently use a script I got here http://www.agtc.co.uk/?menu=scri. The php membership system has worked pretty well for me so far. The adduser.php page which sends out an email to anyone who signs up doesn't really work because depening on your mailer, it won't send to every domain, but if you know much about SMTP and proper mailing, this should be pretty easy to fix (in which case, let me know how you fixed it because I can't figure it out yet). On some machines, this won't even be a problem. Even if you don't want to use this script, it could be used as a pretty good example of how to set things up for yourself.
