Can X run with 16mb of ram though?
I honestly don't know. It is going to be a VERY tight squeeze if it does though, and that may be the root of your problem.
If you're only trying to run X for the fun of it, I wouldn't push this very far. My first Linux box had 49MB or RAM and while it did run X, it wasn't pretty. Web pages took minutes to load, forget about playing a movie.
The joy of Slackware is that it does so much from the command line. Heck, if you want you can even surf the web with Lynx. It's kind of weird, but it works!