Please Help! I Have Lots Of Questions.

In case some of you have read my previous posts, you may be aware that I'm writing a webboard application as a replacement for the old one.

The old one currently have approximately 50000 topics in the database, each has on average 10 replies (I just check recently. I though it was only 7000 topics).

I need to provide paging and sorting feature to the topic list. But I can't just SELECT all of them and let GridView do the paging/sorting, right?
I have been using stored procedures to store my SQL statement for several projects now. I know how to deal with the paging feature (ROW_NUMBER), but the sorting requires me to change to change the "ORDER BY" clause.

1. Can somebody tell me how to change the ORDER BY clause in the stored procedure(s) at runtime? Or does anyone have other approach?

Currently I'm thinking about moving back from store procedures to hard-code SQL statements, and then modify/generate the SQL statement for each paging / sorting. But I've learn that stored procedures give more performance and security.

2. According to the situation I provided, is it worth moving from stored procedures to hard-code SQL?

I'm also using 3-tier architecture approach + OOP. But I reach a conflict in my thoughts.

You see, according to OOP, I'm supposed to create classes that reflect the actual objects in the real-world, right? In my case the classes are "Board, Topic, Reply, ...." According to this and 3-tier approach, I intend to use ObjectDataSource as a bridge between Presentation Logic and Business Logic. But I wonder what my datasource class should return

3. Should my data source class return data objects like

1st approach

[DataObject(True)]
pubic class TopicDataSource{
         public static Topic[] GetTopicList() { }
}

or should it return DataSet / DataTable / DataReader like

2nd approach

[DataObject(True)]
public class TopicDataSource{
          public static DataTable GetTopicList() {}
}

Personally I think approach 1 is more OOP and allow for more extendability, but approach 2 might be faster.

4. If I go with approach 1, how should I control which property of my data objects is read-only after it's has been inserted/created?
Can I just set my data object's property to be readonly? Or do I have to set it at page level (i.e. GridView-> Columns -> BoundField -> ReadOnly=True)?
Or do I set it and the page level and write a code to throw an exception in the rare case the application / user try to change it's value? Or else?

Please help. These questions slow me down for days now.

If there's any concepts that I misunderstood, please tell me. I'm aware that I don't know as much as some of you.

I will be extremely grateful to anyone who answer any of my questions.

Thanks a lot.

PS. For those who think my questions are stupid, I'm very, very sorry that I bother you.