I am developing a web based application whose primary objective is to fetch data from the database, display it on the UI, take in user inputs and write them back to the database. The application is not going to be doing any industrial strength algorithm crunching, but will be receiving a very high number of hits at peak times (described below) which will be changing thru the day.

The layers are your typical Presentation, Business, Data. The data layer is taken care of by the database server. The business layer will contain the DAL component to access the database server over tcp. The choices I have to separate these layers into tiers are:

1. The presentation and business layers can be either kept on the same tier.
2. The presentation layer on a separate tier by itself and the business layer on a separate tier by itself.

In the case of choice 2, the business layer will be accessed by the presentation layer using a WCF service either over http or tcp.

I don't see any heavy processing being done on the Business layer, so I am leaning towards option 1 above. I also feel for the same reason, adding a new tier will only introduce the network latency. However, in terms of scalability in case I need to scale up or scale out, which is a better way to go? This application will need to be able to support up to 6 million users an hour. There will be a reasonable amount of data in each user session, storing user's preferences and other details. I will be using page level caching as well.