Hi:
Don't first look at tools. I repeat, do not first look at tools. First, think Agile. And remember, that Agile is not just about minimual documentation, it is just as much about quality documentation. In other words think about requirements documentation that captures the essentials and only the essentials.
Additionally, Agile says forget about long winded text based requirements. Instead focus on requirements at higher level of abstraction, and then let the developers hash out the details. Think modeling.
The real need - especially in complex projects - is not so much about capturing the indivdual behaviroal requirements, but the essential interrelationships between higher level requirements. This puts the stongest empahsis on graphical requirements gathering techniques that capture these essential interrelationships. User Stories fail here.
Also, to handle complexity, the BA needs to be guided in accopmplishing a logical, natural decomposition. The typical sledge hammer approarch to systems decomposition will not work with complex systems. User stories fail to guide in proper decomposition.
After such consideration THEN think tools. My favorite tools, by the way, are pencils, paper, white out, and scotch tape - especially for complex systems.
Tony