As Izunya said, Williams uses bits and pieces of music from other composers, but Wagner wasn't one of them, as far as I know. What Williams did borrow from Wagner (and other composers) is the idea of the leitmotiv, which means that the each major character has his or her own "theme" music. There's the theme for Darth Vader (the aforementioned "Imperial March"), and Luke and Leia both have their own themes.

I agree with the mention of Star Trek II as some of the best music put to sci-fi. The whole nautical feel of the music just seems to fit the scope and theme of the movie.