Computer skills are highly valued in japan. For jobs such as webmaster, etc, I believe there is a shortage of skilled labor in Japan because a lot of the newest, most advanced computer programs/etc (I dont really know what I am talking about here, just heard this from others in the profession) are written in English. If you want to make money in Japan, having computer skills is the easiest way (aside from teaching English, which to be honest kind of sucks if that's not going to be your 'career.' For career teachers, more power to ya).
Study when you have time. But realize that you will improve ridiculously fast once you move to Japan, so any time you spend studying back home wont be nearly as effective as time spent in Japan, especially if you dont have a regular course/tutor.