I have an application which I am getting ready to design and I would like to obtain advice from the folks in this forum on this design. Details regarding this application are as follows:

The application will be an inventory system with at least 3000 users. The application's design should, however, be scalable enough to allow for more users in the future without any noticable drop in performance.

It has already been decided that ASP.NET and C# will be the core languages used to develop this system. Taking this, along with the following details into account, what type of architecture would be best suited for this application?

The application architecture should:
Allow for maximum performance for bandwidth (high traffic).
Reduce the number of trips to the server to display the data.
Be scalable.
Be secure.

In addition, I would like to know the following:
What type of architecture would be most effective?
Would you use web services?
Would you use a datagrid, datalist or repeater to display results?