Question about User Stories   

Hi all,

I have just finished reading a number of books on extreme programming and am seeing how I can approach introducing it into my team. I have a question relating to User Stories that I'm hoping I can get some direction on here.
From what I have read, User Stories are created to cover each of the processes and features that a customer wants a system to perform. Each of these stories will be presented in planning games between iterations and can be selected by the customer independant from each other for implementation in the next iteration. Each User Story also has an estimate (in IDDs, points, etc) based on difficulty and effort required to implement.

If what I have understood is right, how do you estimate for User Stories that rely on other User Stories being completed first? For example, if I have 2 user stories for a simple web site, 1.Log in to the website and 2.View your account details. Do I make estimates for these two Stories seperately based on what would be involved in getting them both in place - even though story 2 would include story 1? And if this is the case, as the site gets more complex, if the customer picks quite a deep user story early on in the process, should it's estimate include all of the preceeding works' estimates?

Or do User Stories have dependancies - other user stories that must be completed before offering them to the customer as a choice? This would enable each user story to be estimated based on the condition that the preceeding work has already been completed?

I hope this all makes sense.

Thanks in advance,

Alan