At 768p i'd use most the latter settings still but consider turning up some other details to High such as Ocean quality and reflection quality. Other than that you could try turning up AA to MSAA 2x (Sharper) and AO to HBAO (More realistic and accurate lighting) and that should give you good performance with nice graphics quality. Just experiment for yourself once you have tried the above settings to see if you can handle any more higher details.