To properly install Windows 2008 on a Dell 2800 you need to use the Dell Systems Management Tools and Documentation DVD. This preloads all of the driver support you need
Go to Support.dell.com, select drivers and downloads, enter your service tag, select the OS version you are installing. Then look in the Systems Management Section and follow the directions to download the 2 .iso files needed.
Download all ISO file segments into an empty subdirectory on your local hard drive. After you have downloaded all the file segments, run the following command to create the ISO file:
copy /b OM* OM_610_SMTD_A00.iso
Use ImgBurn.com to create your DVD
Boot the server with this and off you go.