Thanks a lot for everybody's answers! I will wait for the KWifimanager and X modules to appear in the Control Center of 3.2 after some new update. Meanwhile I found the new XF86Config-4 where David suggested, and, voila! 96 dpi again.
Joan (BTW why changing the DisplaySize option is better than forcing a dpi value ? ; also, To which section of XF86Config-4 do I add the line?)
