Forbes.com has published it's list of America's 25 Richest Counties.  This list is based on 2008 median income.  Median is the number directly in the middle.  This means that in these counties 50% of the population makes above this number and 50% makes below this number.  Of the 25 counties on the list, 11 are in the DC area (6 are in the top 10).

1.  Loudon County, Va - $110,643

2.  Fairfax County, VA - $106,785

3.  Howard County, MD - $101,710

6.  Fairfax City, VA - $98,133

9.  Arlington County, VA - $96,390

10.  Montgomery County, MD - $93,999

12.  Stafford County, VA - $89,536

13.  Calvert County, MD - $89,049

14.  Prince William County, VA - $88,675

21.  Charles County, MD - $86,586

23.  Alexandria City - $85,135

In total 19 of the 25 counties were located on the East Coast.