The Casey lines of South Carolina originate from Virginia according to family tradition. These South Carolina Casey lines have many DNA submissions and all but one line tested to date has been found to be closely related. There are also several Casey lines from Virginia tested as well - but none of these are related to the South Carolina lines to date.
If you have your Casey DNA tested, you will definitely learn if you are related to the many Casey lines in South Carolina (around 40 lines that can not be connected with traditional documentation to date prior to 1800). Or you could match one of the existing Casey submissions that have Virginia ties. To test, I would start with the 37 marker Y-DNA test from Family Tree DNA. Be sure to join the Casey surname project first, then immediately order the the test which will save you $20.