The next upgrade requiring you to purchase a new bicycle will be the CVT or Constantly Variable Transmission. One system is contained in a sealed hub and consists of two cones with a drive ball between them. Here is a link to this system.
http://www.varispeed.com/news.htm
The other system consists of a large disk and drive cone. This system is exposed to the elements and is unsuitable for MTB applications and must be kept very clean for proper operation. The only advantage with this system is its cost compared to the sealed hub system.
http://www.cvtsystems.com/gearphot.htm