Updating to eCart 6 won't help with this feature at all.
There are two ways to try to achieve this... data mining of actual orders to see what users have also bought, and adding something to the admin back end that allows an administrator to select additional products to be displayed on the product details page.
It is probably easiest to do the first, since the user experience to allow an administrator to select and associate products can be complex to pull off in a user-friendly manor. The former is just a matter of writing a complex SQL statement and adding a recordset to the page to show what customers have actually bought, so no additional tables or admin controls are necessary.
Did this help? Tips are appreciated...
