This depends on how many hits your site gets. The data collection code is very light weight, however the reporting and statistics code can take a lot of memory to process. The longer you collect data for the more memory you will need to process it. At a bare minimum, a basic WordPress site with WP Statistics should have at least 32 meg of RAM available for a page load. Sites with lots of plugins and high traffic should look at significantly increasing that (128 to 256 meg is not unreasonable).