I currently own the DGL-4500 so this is coming from an experienced user of 2 years with it and 4 years of the DGL-4300.
There are two ways ports work incoming and outgoing. Now if you you want the router to port forward certain ports for games to multiple machines for inbound connection know this is not possible to do with any router. This is because you can only forward one port at a time to a computer behind that particular router with an inbound connection. An example of this would be you can't run a TF2 server on 2 different computers with the same port they have to be different.
There is no limitation for multiple machines to use the same port with outbound connections. An example of this would be port 80, that is the default port for HTTP so you are always sharing that port when there are multiple users on actively using the internet. This rule also applies to games, to take stab in the dark are you having a problem connecting to server after one of you has all ready connected?
If that is the problem then the problem is not on your end, this is the servers. This is because they want to prevent cheating or what ever the reason maybe, they limit the amount of connections with your external IP. This is a pain when you want to play on a server from the same location, had this happen a few times myself.
Now about your lag issue, what is your connection speed? Upload and download please. Also, whats your location and the servers location?
Is this router going to reduce your lag? Yes it can but only with local traffic, it will not do it automatically however. There is a feature called Gamefuel, you get to it by going to Advanced > Gamefuel and there you will see an option "Add GameFuel Rule" this is where you need to enter your info. The IP of the server you want to connect to goes in Remote IP Range, set the priority to 1 because that is the highest, then 2 for the next one and etc. The Local IP Range is your network IP so if you want every one to benefit from that server not just one computer, put in the whole network range 192.168.0.2 - 192.168.0.255, now if you want one just put in that comps IP only in both boxes. The Local and Remote Port Range HAS TO BE THE SAME, if they are not this will not work, also under Port Range (drop down) just say Any, this way both UDP and TCP are covered. For the name I just usually punch in the server name and the name of game. Lastly make sure you check Enable before you add it at the bottom, this way it will be active when it is added.
The one thing you need to know about Gamefuel is that it will only prioritize your traffic from behind that router, so if you have a router from your ISP and someone is plugged directly into it and the rest of you are behind the DLink it will not limit their traffic and not give you the best speed possible, especially if they are watching youtube or downloading. This will not speed up your connection with your ISP either, this will only make it so your local traffic is giving the best speed of your available connection to the servers you setup in the router under Gamefuel.
There is also something else you should know about that router and that is there is 2 separate processors, 1 for the LAN/WAN and the 2nd for the WIFI. This is important for all users because when they connect they have to wait for the router to respond to the connection attempt and that sometimes can cause the router to stop all traffic for a few seconds, that wont be a problem with this unit. Therefore if some one games through the WIFI they will see an added benefit, along with those across the network. Now I don't recommend gaming on a WIFI connection but sometimes circumstances dictate the situation rather than whats preferred.
