How to start a multi site web application

I am trying to plan an architecture for an upcoming web application that involve a few geographically separated server sites. For example, a group of servers will resides in New York and the other will be in California. My problem is regarding the data part of the application. Each sites will have their own group of database servers. How do I keep the data to be consistent? Also, how to make the New York server to serve the clients in New York and California server fro clients in California. Is it through the ISP? BTW, I am using the SQL Server 2005 and ASP.NET.