We also had trouble using PXE boot with the 755 and 780 models. In our case we are using FOG for imaging and including ATI Radeon video cards to support dual monitors. The PXE boot process would fail with the ATI card in place. Pull the card and the image works fine. Very inconvenient, but works for a quick solution. All of the prior tricks with BIOS updates and settings didn't work for us.
I had to download the generic driver for the Intel 82566 -- see http://www.intel.com/support/ethernetcomponents/controllers/83566/sb/cs-029661.htm
PXE works fine now.
The posted Link has been broken for over 5 years.
Where can I get drivers?
Download generic drivers for Intel® 82566 Gigabit Ethernet