advantages of using a middle tier. help please

Hi all

we are about to start designing a desktop application that will be written in C# with SQL server as the back end and we would like to get your idea on the best way to design this thing.

the main purpose of the application is to report financial data in a fancy way. we are going to be using a lot of datagrids (hundreds of rows) and a lot of coloring and fly overs.

the users don't want to use the web as the front end, so we will have it as a stand alone application.

the number of users for this application, will not exceed 10 and we would like to have a very good performance with this application; keep it as fast as possible.

will it be of an advantage to use a middle tier or just do data preparation in the Database itself using stored procedures and then link it to windows forms. will it bring the performance down if we use a middle tier. in other words, which one will be better, write code that connects directly to the database and do data massaging in the database, or write a middle tier that will take care of that. what will be the form of this middle tier, webservices?

It will be great to get your opinion. if you have more questions, please let me know.

thanks in advance for your help