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.

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.