My old bike was a 2120 too, with slightly upgraded wheels (low-end Rolfs) but otherwise original components. My wheel upgrade was really a mistake -- the Rolfs were much stiffer (and only a bit lighter) than the originals, and made for a *very* harsh ride. I thought about upgrading to a carbon fork to help smooth out the ride, but was also never happy with the 105 derailleurs. While contemplating an upgrade to fork and components I fell in love with an Airborne Zeppelin. So I got $400 for the old 2120 and a beautiful new ride.

But, I'd agree it's best to demo a few other bikes.