3GS cracked screen. Do I need the lcd too?

you can buy a replacement digitizer with frame and use your original lcd... Remove the screen by following a guide from ifixit.com, unscrew the lcd from the frame, put it in the new frame, connect the connector back to the logic board and gently push the screen back down, put the 2 screws back in the bottom and your done.

if you have only the glass digitizer it will be more of a complex job as you will need to remove the old broken digitizer from the frame which is stuck in with adhesive.

I personally would recommend you buy either a new frame and adhesive and stick the new digitizer into the new frame and transfer the proximity sensor over to the new frame or buy a half front assembly for your iPhone which will be everything apart from the lcd.

Heres a guide incase you decide to do the more complex job - iPhone 3G Front Panel Replacement

and heres a guide if you decide to buy half front assembly - iPhone 3G Front Panel Assembly Replacement

just follow the second guide in reverse to once disassembled your device to put the lcd into the new frame.