If you're serious about your business there's only one system to look at.
Elevated X has already been mentioned here but don't take our word for it, do your own research. Do a gfy search for CMS related threads over the past year or so and see which system is recommended the most by actual users.
Feel free to email, ICQ or give me a call at 800-690-9277 and I'll be happy to answer any questions you have about our system. building and running pay sites and content management systems in general
AJ