I also agree that you should first analyze exactly what features are most important to you and then look for a tool. Also, I'm starting to see a distinction of requirement definition tools and requirement management tool. A requirement definition tool will help you elicit and document your requirements through modeling, process flow diagrams, prototyping and use cases. A requirements management tool will serve as a document repository and allow you to store, trace, and version your requirements. Take a look at a tool like Blueprint for the next generation requirement definition tool or IBM's new Rational Composer. For requirement management tools you can checkout IBM's Rational Reqpro.
Check this one out.
http://www.sparxsystems.com.au/products/ea/index.html
We are just going into pilot on a hosted application http://www.brightgreenprojects.com that simplifies the process of capturing business requirements, along with risks, issues, meeting minutes and change logs.
As a pilot user and we are offering six months free usage in exchange for regular feedback on any required modifications for later release. Your Organisation sounds ideal for our solution - if you are interested in finding out more have a look at the site, you can also register your interest in the pilot programme.
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