Requirements management software

Hi guys,

I would like to discuss a certain aspect of project collaboration - requirements management.

When I have to deal with a large project it becomes hard to manage all the data via email so I need some service to help me organize/discuss/refine information I receive from my customers.

There are well-know tools like Basecamp but they do not emphasize requirements management process. I.e. I can get some kind of trivial feature like Writeboard in Basecamp or general purpose wiki in Redmine but I always wanted something built exclusively for RQ management.

Here is a list of my wishes to such software:
1) Web based hosted solution (so I do not have to install\maintain anything)
2) Relatively cheap ($15-$30 per month will do)
3) Keeping RQ document as a list of separate RQs that can be referenced by unique Id
4) Keeping history of changes
5) Fast and accurate full-text search through all RQs/versions
6) Tags/labels for RQs
7) Baselining of RQ document (i.e. a snapshot of the current RQs list)
8) Built-in chat/comments system to negotiate on RQs with customer
9) Integration with email
10) Ability to store attachments
11) Ability to export a list of RQs to PDF or Word document
12) [Most important] Extremely simple UI so non-techie person (my customers) can use it

To summarize I need something like Basecamp but targeted specifically to requirements gathering. I.e. I need to easily collect/organize/discuss customers wishes. There is no need for built-in issues tracker, calendar and other features commonly found in Basecamp-alike services.

Here is my questions to you:

Do you know any software that matches my requirements?
What tools besides email do you use to collaborate with the client?
How do you keep your project requirements?