I thought it was a easy answer, but after some digging around, your right. This is a toughfy. I found this post http://forums.devshed.com/networking-help-109/multiple-computers-playing-world-of-warcraft-333793.html
which I will repost here since I find it pretty helpful:
Your setup is correct, the problem is not on your network segment.
I certified with A+, Linux+ and Network+.
I play WoW also, and I have lag, just as you do, same times of day
My setup is significantly more hardcore, with a dedicated PC running BSD acting as my gateway, and I still lag out at peak times.
Why is your setup correct?:
WoW's client only sends data along a stream that your computer requested to open. When you sign on to their game, your soliciting their server(s) to initiate a connection. A computer served by a NAT connection, will connect to their server(s) fine. A solicited connection.
Port forwarding is the exact opposite. You are explicitly enabling any outside connections, welcoming unsolicited data. Port forwarding (typically) only works with one computer because your telling the gateway to 'Look if someone wants to access my webserver its here on my network'.
The Port forwarding stuff Blizzard is talking about concerns those that aren't running some kind of SPI (State full Inspection Firewall), on a hardened network. NAT is intrinsically a SPI. Blizzard's client won't work on a networks hardened against solicited data, i.e., block everything but port 80. Chances are, unless your at work, your network isn't hardened to block any outgoing data. Most ISPs don't block outgoing data, or solicited incoming data. On a hardened network, the WoW data is blocked from leaving the network.
In your specific case TheAaarrrggg, your router is NAT & SPI enabled, page 40 of its manual found here ftp://ftp.dlink.com/Gateway/dgl4300/Manual/DGL-4300_manual_06292005.zip
With 2+ computers each computer opens its own connection, which your router keeps track of.
Blizzards CLIENT, by design must be functional with NAT, or the teeming millions of players would have to have IT experience to get their game working properly, not mentioning having to call their ISP to tell them to 'oh btw open some port'.
Now the downloader is a different story.
NAT sucks, when you want to have a computer act as a server, like when downloading files via Blizzard's downloader. The downloader can work behind a firewall, albeit slowly since its designed to let other customers download data from you. (Does this sound like an unsolicited connection from who knows where with your computer acting as a server? Exactly.) There you need port forwarding for it to work as fast as possible. But it still works without it.
So what does all that mean? 1 computer playing WoW, doing port-forwarding may or may not make anything better, but won't hurt anything either. 2+ computers playing WoW, you don't use port-forwarding.
192.168.0.1 > Advanced > Firewall > NAT ENDPOINT FILTERING
You need to set your firewall to UDP=Address UDP=Endpoint. If that doesn't work, you may have to set up some port triggers, but that isn't going to be easy, though it probably won't be needed. Let me know how it goes.