Get this book:

http://www.amazon.com/gp/product/1430228865?ie=UTF8&tag=aspnettelligent-20&linkCode=as2&camp=1789&creative=9325&creativeASIN=1430228865

Then do it in MVC2 (MVC3 is coming out soon with some useful improvements too).

ASP.NET web forms is a bastardisation of the internet. ASP.NET MVC is a pure (more or less) standards-compliant and standards-embracing way of creating segmented tiered web applications.

Using something like Microsoft Practices Unity, or MEF (the Managed Extensibility Framework), you can make it totally modular, build it in slices of functionality, and MVC is very very good at supporting unit testing.

That book is the best IT book ever written. FACT.