I'm a newbie also, but I noticed a slight power increase from the replacement of the stock muffler.  My 2001 has a Jardine slip-on,(sorry, can't remember price) and it gives it a little more power, for little effort.  Gives it a nice rumble, too.  
Also, even though it doesn't make it go faster, a windscreen makes for a much more pleasurable ride.  Especially if you live in the country with lots of bugs.