I have OLTP central database sql standard 2005 that is currently working within LAN. It is required that new three remote branch sites will connect to the SQL server and do update/insert/delete and reporting. The four sites are connected with frame relay WAN. The main site is 512 kbps and has the database...

True, you can setup a log shipping or Snapshot Replication for this purpose, BOL has almost every details you would be needed to set it up though here is a link to an article on http://www.sql-server-performance.com/articles/clustering/log_shipping_70_p1.aspx and one on Snapshot replication http://www...

Fair enough, in this case you can take help of log shipping (manual) to perform updates between central repository and client's end (laptop). Also replication will help you to update only required tables. If the client's database is READONLY then simply you could schedule a manual job to backup...