As a business analyst, I spend a lot of time focusing on gathering good requirements. Once I have a high level set of requirements, I work to refine the requirements to the point where the developers will have enough information for design and implementation of the application. Everyone has a method for achieving this requirements definition process but I would hope at the end they have a fairly robust set of requirements from which to work.
This is where I find myself at a loss when working in an Agile environment. I know that only the user stories that are being worked in the current sprint are expanded. These user stories encompass a subset of requirements for the project. But at what point do those of you doing Agile find the simple 3x5 cards become unwieldy? At what size project do you start to say "Maybe we need something to keep track of these user stories"? And more importantly, where do you turn when you reach that point?