snuffi

Подписывайся на событие об изменении коллекции

private void DataGrid_Loaded(object sender, RoutedEventArgs e)
{
var dg = (DataGrid)sender;
if (dg == null || dg.ItemsSource == null) return;

var sourceCollection = dg.ItemsSource as ObservableCollection;
if (sourceCollection == null) return;

sourceCollection .CollectionChanged +=
new NotifyCollectionChangedEventHandler(DataGrid_CollectionChanged);
}

void DataGrid_CollectionChanged(object sender, NotifyCollectionChangedEventArgs e)
{
//здесь пишешь в базу
}

пользуйтесь классами вместо DataSet, и вычисляйте внутри классов все что угодно

вот хорошая библиотека json.codeplex.com
ее api: james.newtonking.com/json/help/index.html

пример использования:
Product product = new Product();
product.ExpiryDate = new DateTime(2008, 12, 28);

JsonSerializer serializer = new JsonSerializer();
serializer.Converters.Add(new JavaScriptDateTimeConverter());
serializer.NullValueHandling = NullValueHandling.Ignore;

using (StreamWriter sw = new StreamWriter(@"c:\json.txt"))
using (JsonWriter writer = new JsonTextWriter(sw))
{
serializer.Serialize(writer, product);
// {"ExpiryDate":new Date(1230375600000),"Price":0}
}

private void Form1_Load(object sender, System.EventArgs e)
{
// Bind the DataGridView to the BindingSource
// and load the data from the database.
dataGridView1.DataSource = bindingSource1;
GetData("select * from Customers");
}

private void GetData(string selectCommand)
{
try
{
// Specify a connection string. Replace the given value with a
// valid connection string for a Northwind SQL Server sample
// database accessible to your system.
String connectionString =
"Integrated Security=SSPI;Persist Security Info=False;" +
"Initial Catalog=Northwind;Data Source=localhost";

// Create a new data adapter based on the specified query.
dataAdapter = new SqlDataAdapter(selectCommand, connectionString);

// Create a command builder to generate SQL update, insert, and
// delete commands based on selectCommand. These are used to
// update the database.
SqlCommandBuilder commandBuilder = new SqlCommandBuilder(dataAdapter);

// Populate a new data table and bind it to the BindingSource.
DataTable table = new DataTable();
table.Locale = System.Globalization.CultureInfo.InvariantCulture;
dataAdapter.Fill(table);
bindingSource1.DataSource = table;

// Resize the DataGridView columns to fit the newly loaded content.
dataGridView1.AutoResizeColumns(
DataGridViewAutoSizeColumnsMode.AllCellsExceptHeader);
}
catch (SqlException)
{
MessageBox.Show("To run this example, replace the value of the " +
"connectionString variable with a connection string that is " +
"valid for your system.");
}
}