You'll certainly want to pay them a visit and learn more about Cassidys past and present. Our Cassitys and quite a few of the families associated with them in early Virginia and Kentucky were of Scots-Irish descent. You'll want to check out this site to learn more about them. You can look for general genealogy help at USGenWeb or go to individual states' pages. On the state pages you can link to each county's webpage to see what help you can find there.
Some counties have birth, death, marriage records, wills, census lists, etc.
South Carolina . Links to other states can be found at the USGenWeb site. This is an amazing compilation of genealogy URLs sorted by categories. Here you can post queries on pages devoted to each of your surnames or on pages devoted to some various topics of interest to family researchers. Rootsweb Here you will find links to mailing lists for US states, areas and individual counties, foreign countries, surnames, ethnic groups, genealogy software and more.