What are my WPMU membership parameters?

Hi guys. I am trying to launch a new directory website. Currently, I run tutuology.com. It is hosted through pSek and I use WPMU. I am a pro member.

For the new directory site, I'd like it to be completely separate from tutuology.com. Here's why: if anything catastrophic ever happens to tutuology.com, I do not want the directory site to be affected. So, my question is: do I need to purchase another wpmu membership or can I use my existing membership?