Another place worth checking out is the Korean film archive. The website is www.koreafilm.org and is in English with directions to its various locations.
They show older Korean films and some foreign films, although unfortunately the Korean films are usually not with subtitles. This is a problem in general in Seoul; if you want to see Korean cinema you're usually confined to DVD, at least in my experience so far.
The archive also has a library with a decent selection of film books and journals. It is not as extensive as a ordinary university library in North America, but it does have some useful material. It costs 30,000 won for a membership in order to borrow material, but there's no entry fee.