Not being able to un-check it means your GPU shares memory with the computer (in most cases), and is not using dedicated memory, and IE9 will automatically enable hardware acceleration where available. You can test this by trying some of the IE test drive sites (Fish Tank comes to mind as the beta version of IE would only go to about 20 FPS without HA, and a full 60 FPS with HA. Now without being able to un-check the box IE gets around 40 FPS, so HA does help where it can). For the most part you will see a good overall improvement on graphic intense sites.