Microsoft’s greatest triumph with the Xbox 360 is undoubtedly its Xbox Live service, buoyed by robust selection of downloadable games. You can buy digital versions of classic arcade titles, original Xbox games (and starting next month, Xbox 360 games), avant-garde indie gems and even some Nintendo 64 classics. Customers are so in love with the service that they haven’t even noticed that prices for downloadable games have crept up more than 15 percent since the system debuted in 2005.