Most all major hosting companies provide minimum of 64MB of memory, but every hosting company and package is different. If you are using PageInvasion and trying to build up to 5000 pages and have less than 64MB of memory you will need to upgrade. If you are not sure, check with your hosting company.
For instructions on how to increase the size of your server hosting click here: http://www.ducea.com/2008/02/14/increase-php-memory-limit/
If you have any technical questions about memory requirements for PageInvasion or other technical questions please contact support at support[at]pageinvasion.com