We are in the process of defining the minimal requirements, but in the meantime you can use the minimum requirements of Joomla as our idea is that you should be able to use JoomlaStats on all Joomla CMS installations:
This can be found on Joomla 1.5 and 1.6 technical requirements or Minimum System Requirements.
P.s. If you have a weak machine, then you might want to do an installation from directory.