Paul,
you can add NIC drivers to a current image.
im not sure what the folder structure on your server is, but you will have something like this:
c:\RIS\setup\english\images\DellPCs\....
at this point add the following folders (if not already there):
\$oem$\$1\drivers\nic\ and copy the drivers in here.
ALSO add the drivers to the following folder (again im guessing your folder structure)
c:\RIS\setup\english\images\DellPCs\i386\mirror1\userdata\windows\
That should work.