Members Only? - plug-in or application suggestion

[Ritarun]

I will be building a WP site for a non-profit organization with paid and public member categories that needs to be restricted to only paid members of the organization for some sections and public viewing in other sections. This organization has a traditional web right now but wants to change to a more interactive/blog style format. It currently has a special script/application that offers page and folder protection.

Being new to WP I am not familiar yet with recommended tools that would work that could be added to a Revolution themed WP blog. Are there any tools like this for a blog? plug-in?

If so - recommendations and or links with examples would be appreciated. Thinking of using either the Office, Lifestyle or Chrome theme but I need to play with them more before I decide what might work.

Thanks....