As far as upgrading components goes, it is very expensive to upgrade the gruppo. When buying a new bike, you're basically buying the components and getting the frame for free. Therefore, it is usually a better option in the long run to purchase a new bike then significantly alter yours.
If you are not willing to spend that much, however, a new set of wheels will really liven up your bike and give you the actual performance boost you are looking for. Replacing wheels, for several reasons, is the most cost-effective way to upgrade your bike.
Look at one of the top-end wheel manufacturer's entry-level wheelsets. Everybody has their own preference when it comes to wheels, but to give you an idea of what level to look at, I would suggest something like the Mavic Cosmics, Velomax Circuits, or Cane Creek aeroheads. This will be a $400 upgrade that will do wonders to your ride and speed.
