I have just purchased a Gigabyte GA-X58A-UD7 motherboard, and it has dual Realtek RTL8111D gigabit LAN ports. I also have an Intel PRO/1000 MT PCI gigabit network card lying around from an old desktop. I've heard that Intel PCI-E network cards will often outperform Realtek, Marvell, or other non-Intel on-board network adapters, but my question is will the PRO/1000 MT PCI card outperform a single Realtek RTL8111D gigabit network adapter in terms of transfer speed, or should I keep what I have/buy a PCI-E Intel network adapter?
If the Intel Pro/1000 MT is the "Server" edition, then it should outperform any on-board NIC due to off-loading (the server cards process data on the card, freeing up your CPU to prepare the next bit of data).
I dug through Intel's site a bit, and it seems as the Pro/1000 MT "Desktop" edition may also support a few types of off-loading in the advanced settings. But the documentation is sketchy as the card is discontinued and grouped with several other cards.
If those options are available to be turned on in the advanced settings of the (full) Intel NIC driver set, then I'd bet on the PCI MT Desktop adapter for sure. :)