My target area is in Fort Mill, SC, but I want to reach people moving here. I should use United States as my area preference instead of the targeting the local area, right?

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I would create an ad campaign that targets the NC and SC area. This
will help you keep your costs down, because this market is competitive
and will make sure your ads are target appropriately. I recommend
using Google Analytics for a complete picture of where your visitors
are coming from geographically .

I also recommend that the keywords you bid on have the geographic
areas of your areas whenever possible like, Homes in Charlotte,
Homes in South Carolina further increasing your relevancy.

Finally, Googles technology can Identify keywords such as
Charlotte or North Carolina and serve then even if the searcher is
outside of your targeted geographic region. So if you were running an
ad campaign that just targets NC, but someone in Vermont searched
North Carolina Homes, a keyword you were bidding on, your ad should
appear. For further info on this topic:
http://adwords.google.com/support/bin/answer.py?answer=35435&topic=7065