Can you provide more info in the number of requests you get say in a hour or day?
What hardware have you got currently?
Where is the bottle neck currently? Is it CPU or I/O?
ASP.NET can use all cores.
MS SQL 2008 can use all cores and any amount of memory you allow it to.
The problem is never with the Server software but rather always with the code your write (that is your ASP.NET application) and the way you've designed your database (both the logical and physical database) and the queries you write etc.
From experience with optimizing large public domain websites I can tell you that your web server is not where things are slowing down. It is typically in the way the application is designed and coded and how the database has been designed.
You should gather a lot of data on usage, of your current hardware and profile your asp.net application and the database to see what causes the the stress.
Once you know what causes the stress the solution might just be that you need to clean up your code/database a bit.