To be able to use any date, and based on that date, get the start (Sunday) and end (Saturday) dates, use the following T-SQL:

Start of Week (sunday): DATEADD(wk, DATEDIFF(wk, 6, @mydt), 6)
End of week (saturday): DATEADD(wk, DATEDIFF(wk, 5, @mydt), 5)

To test it, just add the following to a new sql query in SQL Server:

Declare @mydt DateTime
Set @mydt='8/4/2010'
Select DATEADD(wk, DATEDIFF(wk, 6, @mydt), 6) as StartDate, 
  DATEADD(wk, DATEDIFF(wk, 5, @mydt), 5) as EndDate