Again about search engine optimized urls

There is a fine article that was recommended to me by baggeroli: http://www.mattcutts.com/blog/dashes-vs-underscores - it talks about what to put in your urls: underscores or dashes. It seems I need to change the urls again from something like English_Spanish_Lessons.html to english-spanish-lessons.html

It's good that the site is still new and there are not much people have already created links to it's individual pages. So it seems that I have to change it ASAP.