It's only 37 minutes on the rapid train to Mitake and a then a 5 minute bus ride to Museo D'Art Ghibli and very much worth the side trip from Tokyo if you're a fan of Miyazaki or animation. If you don't know who Miyazaki is then don't bother. Lots of interactive displays for the kids. All with a slightly retro feel. Nothing computerised here but that is done on purpose. There are some excellent displays of artwork and animation notes from the movies. You can thumb through pages and pages of hand drawn film directions. One display has all the paperwork from the most recent movie and it is a stack about the size of a small car. There's a reproduction of Miyazaki's office which includes ash trays chock full of cigarette butts.
All in all there is about 2 hours of memorabilia and displays in the museum and the ticket price includes entry to the theatre to see a Studio Ghibli short film.
I can't recall any notes on displays or memorabilia from the gift shop in English but everything is pretty much self-explanatory anyway. Hand painted animation cells are available from the gift shop for an average price of 40,000 yen (about $450).
Here is a picture of Totoro and some pictures from outside the museum (you can't take photos inside).