There are 2 basic ways,
You can use broadcoms networking "Broadcomm Advanced Control Suite 3" utility which you should have on your dell support disks, or is available on both suport sites (Dell and Broadcomm) Once in there you can click device managment and the configuration tab and you will be able to set it.
Alternativly if you can locate the devices in device manager you can go to the properties of the NIOC and click the advanced tab. From there you should see a list of attributes you can change one of them being link speed and duplex. Select 1000mbps/Full Duplex and you are on your way.
Once you apply the settings your connection will drop and renegotiate. If you are not absolutly sure that the switching device you have them plugged into will support GigE than be sure you are on a local keyboard monotior setup, as RDP will drop if you do not have a switch that supports GigE on the ports you have them plugged into.
