I was able to install Windows 7 64-bit this weekend on my DN2820fyk with no issues. I used a USB 2.0 install stick, updated to the latest bios first, and selected Windows 7 as my OS in the bios. I installed the latest drivers for everything. It seems to be completely stable and everything appears to be working, but I don't appear to have USB 3.0 support in Windows (the USB 3 controller is listed as an unknown device) and I have a PCI Communication Controller listed under unknown devices.
Performance is excellent and the new video driver works great.
Jason