I just created a new free mod that allows you to set up a cron to regularly clean out your X-cart stats tables:
http://www.bcsengineering.com/store/product.php?productid=348&MMCF_xfCleanStats2
Many of our clients have been using this for years. This allows you to keep your Advanced Statistics on for bestsellers module but keeps the stats tables trimmed down for better performance on the site.
Carrie