I recently got an E6500, and the Dell and Intel NIC drivers would not detect the gigabit NIC. I installed Windows Vista and the driver installed just fine so I knew it was not a problem with the actual physical NIC. The solution was to force the NIC to use a specific driver. Run the driver installation process as normal for the driver from Intel (
http://downloadcenter.intel.com/filter_results.aspx?strTypes=all&ProductID=3003&OSFullName=Windows+Server*+2008+Enterprise+x64&lang=eng&strOSs=174&submit=Go), the Intel 82567 Gigabit Ethernet Controller. The installation will display an error message that an Intel adapter could not be found but the drivers were installed onto the computer.
After installation, start Device Manager (devmgmt.msc) and right-click the unknown Ethernet Controller and select Properties. Select the Driver tab and click the Update Driver button. Select "Browse my computer for driver software" and on the next screen select the "Let me pick from a list of device drivers on my computer." Select Intel as the manufacturer and for the adapter select "Intel(R) 82567LF-2 Gigabit Network Connection" then click Next to complete the adapter's configuration.