dual LAN. WTF is the purpose ????

Dual LAN = Well..."dual" LANs...

Right now, you only have a single LAN, so, no benefit. Now, you could go crazy with LAN configurations....for e.g. seperate all your media traffic on one LAN, internet on the other...or run a teamed adapter setup, or run a VPN on the second NIC or...Well, you get the idea.

Both my servers have dual LAN. One is a commercial (vs desktop) board with two on MB intel NICs. I can use "teaming" function in Windows7. The new server box I just brought up has a PCI Ralink NIC plus the MB NIC and I didn't see teaming function available. Still looking at how to do teaming.

Teaming doesn't really offer you anything unless your routers and switches support Link Aggregation

And even then, it's really useful only on the server side, where you may have multiple clients connecting to it. A single session on the client side will always be limited to the speed of a single NIC.

You most likely have no reason to use both NICs. Some reasons why one might:

1) Link aggregation - LACP: Link the 2 ports together so they act as one really fast connection. Your switch has to support and be configured for this and that's typically only found on more expensive managed switches. This only makes sense when you have clients who together can exceed the bandwidth of one 1GbE port. Unless you had a bunch of computers pounding your server all the time and 1GbE connection really isn't fast enough, this doesn't make sense for someone at home. Corporations and data centers? Used all the time.

2) Link aggregation - Failover: Instead of linking them to act as one port, the second one takes over if the first one fails. Useful in enterprise situations where you can't tolerate a NIC failure. For home users, it usually doesn't matter.

3) You need to connect to 2 different networks. Again, not typical in a home environment but common in corporations and data centers.

4) You're running virtual machines and you don't want them to share a single physical NIC. You might, instead, want them to have their own physical nics.

Just leave the second one disconnected. Honestly, I'm surprised I see so many consumer grade motherboards with 2 NICs. It's just so rare in a home environment.

I have 3 HTPCs that can access the server at the same time. In any case, the server board has this function and I'm using it. This is a BCM RX67Q made for commercial use. It doesn't have the dual NIC issue with intel boards and WHS2011.

From what I can see, when streaming, the HTPC doesn't exceed bandwidth of a N WiFi link, which is less than 300Mb. So even with 3 HTPCs, they will not exceed 1Gb bandwidth in my network.

Nothing you are "streaming" from your server exceeds 50mbps. That's the max bitrate for a bluray, everything else is lower.

File copy is a different issue, but streaming...you'll be fine with a gigabit connection for up to 15 clients, if not more.

The real answer: so the marketing people have a bullet point to check off

Dual Lans are a marketing gimmick for HTPC

That said, I am experimenting with LACP between the server and switch and in between the switches to reduce network latency

My HDHomerun is very finicky and it uses the unreliable datagram protocol to send video to the server running the PVR. it starts tossing frames when there is the slightest hint of congestion. Lately it became worse as my Macs got more aggressive with the timemachine backups, this caused occasional spikes in network latency because of backup traffic. LACP aggregated between the switches and the server seems to help, the frame drops have disappeared. I might make this permanent.

I have dual NICs on my EVGA motherboard in my HTPC. One goes to the House VLAN on my "core" switch, the other is connected to a small VLAN for my HDHomerun. Probably overkill but does work well.

Yup, I like to isolate the HDHomeRun traffic to the server as well since none of my client machines require direct access to them. Otherwise watching live TV on a client would result in double network traffic (HDHR -> server and server -> client). I run a separate switch for them instead of VLANs though.

Not yet. Since Thanksgiving, I pretty much blew my Christmas budget on the wife and kids and had to dip into my electronics budget.

I'm hoping to get new equiptment after the New Year though.

The easiest thing to do is to turn on 9K Jumbo frames on the client and server, it gives me 125MBytes/s on my setup read/write to any client.

If you have already done so, your server software may be holding you back. I suggest you consider Win8 server and Win8 for your workstation, Microsoft improved their SMB file sharing protocol to support native aggregation of LAN links and remote DMA. The former can increase your thruput on multi NICs and the latter halves the network latency, your NIC and client must suport it. Win8 is still pretty new, there are a couple of bumps with it as a HTPC.

If it SSD->SSD you should see 125Mbytes/s which is the Max for GigE if you have a decent nic.

The numbers I mentioned earlier was from my i7950/W7Pro64bit coming off a 2TB Caviar Black to/from my ZFS box

For a server: Either improved performance or improved reliability
For a HTPC: Essentially useless, but it does improve reliability

Besides, you won't have to get a PCI-E NIC or a new motherboard if the NIC you're using dies.