As of the census of 2000, there were 322,895 people, 125,327 households, and 73,692 families residing in the county. The population density was 455 people per square mile (176/km²). There were 131,069 housing units at an average density of 185 per square mile (71/km²). The racial makeup of the county was 77.40% White, 12.29% Black or African American, 0.36% Native American, 6.30% Asian, 0.04% Pacific Islander, 1.04% from other races, and 2.57% from two or more races. 2.74% of the population were Hispanic or Latino of any race. 17.4% were of German, 9.0% English, 8.4% Irish, 5.3% Polish and 5.0% American ancestry according to Census 2000. 87.1% spoke English, 2.7% Spanish and 1.7% Chinese or Mandarin as their first language.
By 2005 non-Hispanic whites were 74.5% of the county population; African-Americans 12.2%; Native Americans 0.4%; Asians 7.8%; and Hispanic or Latinos 3.1% of the population.
There were 125,327 households out of which 29.20% had children under the age of 18 living with them, 46.40% were married couples living together, 9.30% had a female householder with no husband present, and 41.20% were non-families. 29.50% of all households were made up of individuals and 5.90% had someone living alone who was 65 years of age or older. The average household size was 2.41 and the average family size was 3.02.
In the county the population was spread out with 22.10% under the age of 18, 17.10% from 18 to 24, 32.10% from 25 to 44, 20.60% from 45 to 64, and 8.10% who were 65 years of age or older. The median age was 31 years. For every 100 females there were 98.90 males. For every 100 females age 18 and over, there were 97.10 males.
The median income for a household in the county was $51,990, and the median income for a family was $70,393 (these figures had risen to $59,887 and $80,779 respectively as of a 2007 estimate). Males had a median income of $49,304 versus $33,598 for females. The per capita income for the county was $27,173. About 5.10% of families and 11.10% of the population were below the poverty line, including 8.60% of those under age 18 and 5.80% of those age 65 or over.