Shifting from Shimano 2300 to Tiagra
As usual Sheldon has the answer. However you can also find a great range of second hand parts such as derailleurs from your local bicycle shed.

There's little point replacing the entire groupset, however if you want to move up to 9-speed then there's no problem replacing the rear derailleur, cassette and shifters. If you're replacing your cassette, replace your chain.

Sheldon writes:


  In general, all Shimano Cassette Freehubs will work with all Shimano
  cassettes, any number of speeds.


Your crankset, front derailleur and bottom bracket can remain as they are (matched groupsets are a lie sold to everyone by Shimano). If you do decide for a lighter crankset, you can still leave the front derailleur alone so long as it suits your chainring choice. Similarly you can reuse the bottom bracket if it has the right interface and spindle length.