I guess the question to ask is why you want to 'upgrade.' Is it to improve shifting performance, save weight, have a wider/different choice of gears, or some other reason?

If my memory of 8 speed is correct, it uses a Hypergiide cassette, not the older freewheel, so if you wanted you could put a 10 speed cassette, better rear derailleur (105 or Ultegra) and 10 speed shift/brake levers. You would probably also need to replace your chainrings, and of course, your chain, with newer 10 speed ones. Big job that would need quite a bit more than couch change... The shift levers would probably be in the neighborhood of $300 new and about $80 bucks for an Ultegra 10 speed cassette. Then add chainrings and chain, then the cost of the rear derailleur. So you see it adds up real fast.

Last year, I started out wanting to upgrade my bike, but after seeing the cost I decided to just save up some more and get a new bike and I am glad I did.