This is a simple question, but one that varies almost exclusively based on geographical location (cost of living).
According to the US Census Bureau, in 2008, the Median Household Income for the state of Mississippi was $37,818, while the figure for Maryland was $70,482. In California, the average was at $61,017, and in Arkansas it was $38,820.
Since the cost of living is drastically different in various states, counties, and even cities, the figure changes drastically. In San Francisco, California, for example, the median income for 2008 was $71,957, while in Billings, Montana, the figure was $35,147 (less than half as much).
The median household income in the United States is $52,029.