It took a while to get everything straight.
The server is up and running with the 1312 revision of undionly.kpxe
Each dell has to enable Wake on LAN /w PXE and [B]disable deep sleep mode.[/B]
With this settings the setup works for me with the standard configuration.
My project is tested and approved today, so thanks again for the help.
Jeroen v. W.
Wallioo