Two possibilities:
1. Blizzard has always recommended disabling checksum offloading in the network adapter config for the Nforce4, it causes lag problems and issues with chars and npc's showing up as "unknown entities". You may have had this disabled on your previous board, and it would have been re-enabled on the new network adapter.
2. There have been reports of the Nforce SLI x16 drivers not disabling the built in ActiveArmor Firewall, even if the NAM is not installed....causing corrupt downloads and various other issues. The reported solution was to re-install the drivers, allow the NAM to be installed, then disabling the firewall manually through the NAM.
BTW, the only drivers available for the Nforce4 x16 is the 6.82's which ship on the CD....you should not use the std Nforce4 6.70's as the x16 chipset is a combination of the Nforce4 6100/6150 chipset as the Northbridge and the Nforce4 Ultra as the southbridge..