If it's connected to a network, simply open the internal webserver. On the menu "printer information" it show the recognized ram,

Thanks for the info, but actually the printer isn't connected to the network.

Yep,
on the front panel goto menu
configuration / reports and print the configuration sheet.
That contains the installed size of ram

To make sure that your printer is actually using the additional memory (and assuming you are using Windows XP)...go to your "Printer Properties" window, click on "option", see what it shows for "memory capacity"...if it does not show what you expect, click on "Get information from printer".  This last "click" should then change the "memory capacity" to the correct 320mb.  If this does not happen then you may want to re-seat your new memory and make sure it is working properly.