Free and Paid user account

HI,

I would like to setup free and paid membership. I have wpmu - only new user can register, so no new blog. With another plugin I give possibility to users to have acces with one account to all blogs ( blogs are made by admin).

So now I want to setup free and paid membership with fw difrence

FREE membership:
1) Automatically give them Contributor rules
2) In Contributor posts will be automatically PPC ads (cane be this dome be Who Sees Ads for WPMU?)

Paid membership:
1) Automatically give them Author rules
2) In Contributor posts won´t be PPC ads

Do you have Idea how to setup this?

Thanks for an advice
Bigdrobek