I have the exact same bike as you. I have upgraded several things as they wear out, and some just for the heck of it. The biggest improvement I made was getting rid of that Pain In The Arse saddle. I put a Brooks B17 on it and discovered a whole new experience.
Somebody will come on here and tell you that bike is not worth upgrading. I have heard the same thing and don't care. I still have upgraded a lot of the components but mainly only when something wears out. I have over 4000 miles on mine and love it. I also have a Cane Creek Thudbuster seatpost which was a big improvement. Those grips are also pretty crappy, i ditched mine pretty early on.