I'd recommend buying the glass digitiser and front casing complete. I got mine from Ebay (sorry can't post the link). The old glass digitiser is glued into the frame of the phone and it looks quite difficult to get out. Using a complete unit was very easy.
Two things to remember - you have to take the sim card out to get the main circuit board out. And if you change the casing you need to remove the speaker from the old casing and put it in the new one - it's glued in with a rubbery glue and can be prised out.