What Indy said. I'm in that boat. I started riding on essentially your bike. I'm now looking at carbon frame, Ultegra components. (I work for a bike shop. It helps!) I held off on upgrading my bike for exactly that reason. It wasn't worth it, because upgrading to 105 was the only thing that made sense (highest specs on the same frame offered by Giant), but the cost would be only slightly less than buying a whole new bike. Yes, I'll go ahead and upgrade piecemeal, and then replace everything else once the cassette wears out, but that's a while away.