There have been a few people stuck on flashing a Dreambox - this WORKS with a DM8k - can't say for others, but the network bit should be correct.
for the DM8k, you need the secondstage bootloader #73 installed.
You need a PC with a fixed IP address on the same network as the router & DM. Plug the DM into your router, switch it on holding down the bottom button, you will see the IP address in the front display. Now move the cable from the router to the PC, and enter the IP address into a browser. Click on the link 'firmware upgrade' and follow the instructions.
You don't need a crossover cable.