Are you sure you have a 2G? A 1G can only go to 2.1 via iTunes but a 1G can go as high as 3.1.3 that you get here:
To If you do have a 2G that can go to 4.2.1 by:
For 4.2.1 you need iTunes version 10.X and that requires OSX 105.8 or later.
If you have 10.4.11 you can only go to iOS 4.1 since that OSX can on go to iTunes 9.X