http://www.ehow.com/about_5408831_to...igitizer_.html

A digitizer is needed to tell your device where you have touched. So if your touch screen works, you don't need it and you just need the 3.5" LCD.

I suggest you connect your phone to your tv. Download or launch 'Healthcheck' (available from extras-testing). In the app go to 'Tools', 'Accuracy Test' and check that all parts of the touch screen work. If they all do work you don't need a digitizer. If some parts don't work then you need it.

Also some companies sell the LCD and digitizer already assembled. Something to keep in mind for easiness (It's easier to buy them both pre-assembled then messing about with your existing one yourself..).