I tried fixing my LCD and digitizer one time on my iPod Touch and didn't go over so well..ended up breaking the LCD trying to put it in! I would recommend taking it to a professional who knows how to fix it. They aren't that expensive. I got my iPod Touch fixed up for $30.

Ihippo wrote So I cracked my iPhone 4 today, and I'm wondering what I can buy to fox the screen myself.
Digitizer, or LCD?

You can fix it yourself. You will actually need both of those parts but luckily the way the phone is made they are actually one piece. You can go [ Register or Signin to view external links. ] to get an idea of what you need and you can find cheaper sources elsewhere.
The repair on the iPhone 4 is a little extensive but not difficult. Make sure you have a small philips head screw driver, #000 is what i'd recommend, a small flat head, and a razor blade to help get the old screen off. A small spudger/pry tool or plastic knife to unplug the cables help, tweezers are also handy but not necessary.

Need any help just PM me