301 Redirects

[Simon]

Regarding redirecting index.html to the root, it'll be best to specify the page which will be the root index page instead of index.html or you could wind up creating the conditions for an endless loop that you don't want. For example...

redirect 301 /index.html http://www.yourdomain.com/index.php

I'm not sure how to redirect a page or directory that doesn't exist.
Usually that's the job of the 404* redirect, not done by a 301 permanent replacement directive. For example...

ErrorDocument 404 /index.php

Both of those lines go in your .htaccess file. Make sure there's one space where shown, and that you use either relative or complete urls as shown in each instance. I'm just assuming index.php is your new index page so use whatever is correct in your case, but make sure to use the complete url.

Added: If you watch your server's error log for specific pages that get handled with a 404 you can add those to your .htaccess file using 301s in order to capture the PR of those missing pages and pass it along to wherever you redirect via 301 (if you believe it works that way). You can ignore the misspelled urls which get 404s of course.