Testing stored procedure scalability

I have an email application that will be called upon to deliver to the UI the number of new messages for a given user on each page load. I have a few variations of things I am testing on the DB level but all are abstracted by the stored proc call. I'm trying to slam the DB to see what the breaking point (# of requests per second) would be. In a nutshell, I have a table such as this userId, newMsgCount with a clustered index on userId. SQL should be able to server hundreds or thousands of these responses per second. I think the laggard is my .NET app. How can I make this a good test to achieve the test results based on SQL performance? Is there a tool for this that i can give it a stored proc name and param for it to pund my DB? I want to see if the DB can return a min. of 250 responses per second.