|Are you asking about a notebook? If you are, there's no way to upgrade the graphics other than to buy a better notebook. Here's the graphics requirements:|
"DirectX® 9 or 10 compatible graphics card with 64 MB (128 MB or higher recommended) - 256 MB required for HD and AVCHD"
I believe it has to be dedicated memory, not shared. For HD and AVCHD editing, the minimum is 256MB. If you have a desktop machine, you should be able to get a low end card that meets the requirements for about $25.