Driver for NetXen NICs supports upto 31GB of physical RAM on ESX hosts (1003046)
With driver version 3.4.115, physical hosts must be configured with 31GB or less of RAM for NetXen NICs to function properly. This is a NetXen limitation. Contact NetXen for more information about this limitation.
A newer version of the driver is available at http://www.vmware.com/download/vi/drivers_tools.html. Driver version 3.4.350 does not have this limitation on ESX Server 3.5.