Database Commands From C# Form. It Works But ...

Easiest way is to insert one row at a time to database, other method is to query the database first for the data and update, insert the rows and call .Update method on the dataset.  Here is some code sample i copied from VS Help,

public DataSet CreateCmdsAndUpdate(DataSet myDataSet,string myConnection,string mySelectQuery,string myTableName)
{
    OleDbConnection myConn = new OleDbConnection(myConnection);
    OleDbDataAdapter myDataAdapter = new OleDbDataAdapter();
    myDataAdapter.SelectCommand = new OleDbCommand(mySelectQuery, myConn);
    OleDbCommandBuilder custCB = new OleDbCommandBuilder(myDataAdapter);

    myConn.Open();

    DataSet custDS = new DataSet();
    myDataAdapter.Fill(custDS);

    //code to modify data in dataset here

    //Without the OleDbCommandBuilder this line would fail
    myDataAdapter.Update(custDS);

    myConn.Close();

    return custDS;
 }
Refer to help on these topics, you will get some ideas...And one thing to note, In Ver 1.1, even if you do use this method, it internally saves it one row at a time

Cheers 

1) Is it a good idea to not open/close one's connections explicitly?

try to use "using" statement when you create your connection
using (OleDbConnection myConn = new OleDbConnection(myConnString))
{}
(2) Is it a good idea to have not utilized the ExecuteNonQuery() method?
Depends on what you want to do, when getting data use a dataadapter to fill a dataset
or get data into a datareader. ExecuteNonQuery is used to do action sql statments like insert,delete etc
(3) Is there anything that's redundant above? Perhaps the Clear() and Dispose() commands.
see (1)
(4) Would I be paying a heavy price in resources/speed if I were to use Transactions?
not really
(5) My updated values (if at all updated values, else old values) for the database are in a textfile.
Again personal design choice, if you are reading one line from text, and inserting to db, then read next line,
you should think about a different approach. read the text data into a data structure of your choice (ex: dataset)
then do the db operations
(6) Is OleDb passe? Should I go SqlClient?
SqlClient works only with SqlServer

Cheers!