alternate to index.php?

Really quick question here…

I’m pretty fast at installing WordPress and getting the template online and such – but I’m not so super-speedy that there won’t be a little bit of lag time between installing it, putting up a new template set and testing it out prior to making a site “live”.

I was wondering if there was an alternate to “index.php” that WordPress would read? The client currently has a site online, live and working, and usually we’d password-protect a subfolder and to all the stuff in the back (so no one would see) until it’s perfect, then when the site went live (to replace the old) we would just move all the files to the upper level so there’s pretty much *no* lag time between seeing the old site and the new.

You can’t really do that with WordPress (at least, I don’t *think* you can – if I’m wrong, please correct me!) so I was wondering if we could somehow leave the *old* site where it is (it’s in HTML – not PHP), install WordPress in the same level, but not have the WordPress site show up until we were ready for it to? I thought I remembered seeing something in the codex about this, but I can’t find it for the life of me.

Hope I explained myself well enough!