application architecture

I am going to build a web application. Can anyone tell me please, which architecture will be best for performance point of view.
It depends on kind of application you are building. Scalability, flexibility and application type would lead you to different architectures.

for business layer and data access layer should i use seprate project as class library or i should put the code into app_code folder.
General case would be to have separate projects for BL & DAL. This would give you option to deploy them separately if needed and also shield DAL from UI.
But again, based on type of project, if it's a simple small app, you can go ahead with classes in App_Code folder. Though, considering a standard business application, it would be suggestible to keep them separate.