DX is primarily determined by your video card and secondarily by your operating system.
Your 8800GTX is a DX10 video card. Your operating system is Windows 7 supports up to DX11. However, you will not be able to take advantage of any DX11 effects because your video card is limited to DX10. There is no "software patch" you can use; you have a physical hardware limitation.
If you were to have your DX10 card in a system that uses Windows XP, then would will be limited to DX9 because Win XP only supports up to DX9. DX10 and DX11 cannot be installed on Win XP. In this scenario, you are limited by software (the OS) and not your video card.