The integrated graphics(HD 4000) is built in to that CPU/motherboard/chipset. You can download drivers for the HD 4000, but you can't upgrade to it by downloading anything if you don't have the components it was designed for. By the way, until recently integrated graphics was not meant for use in gaming so you'd really want some type of videocard to play the game.
Diablo III is not very demanding so I imagine you could get by with a used card, so as to keep costs down. Something like an AMD HD 7750 or Nvidia 650 Ti is what I'd suggest. You'd want to make sure your power supply can handle the card you choose.
Thanks again!
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