This feature is called "Traffic Shaping" and is one of the best features to look for when distinguishing routers/gateways/firewalls. The way it works is simple. You assign a priorities such as low, medium, and high to a port (or on some more advanced routers an entire port range) and then the router will hold back lower priority traffic to allow high priority traffic to complete first.
This is a great feature to have. For example, I have VOIP Telephone service. Without Traffic Shaping, when I'm downloading the latest linux distro release via bittorrent, my phone might not work well. With Traffic Shaping, I can play games and use Bittorrent at the same time (and still get phone calls).
VOIP and Game (and ICMP traffic like Pings) are all classified as High priority on my IPCop firewall. Bittorrent and Shareaza traffic are classified as low priority. Most "normal" traffic (Web, Email, etc) are classified as normal.
One thing to consider though is the power of the router to handle these decisions. With some lower-end routers I have had trouble with Traffic Shaping when pounding the router with a lot of traffic from different remote sources (such as Bittorrent traffic). Of course, some of the low-enders (Netgear and D-Link specifically) get creamed no matter what with that kind of traffic. Also, low end routers often have limits on their traffic shaping capability. They often only allow you to assign a handful (4-5) of ports as high or low and automatically assign everything else medium. (I've had two Linksys routers that worked like this).
Again, I would consider getting a solid switch (which are mega cheap, Linksys or whatever would be fine) and running an
IPCop (or
Monowall) firewall as the gateway/router. They are easy to use and setup, and offer far more power than any of the sub-$120 routers you can buy out there.
Barring that (for whatever reason) I would look at
SMC routers (the Barricade brand). They have a reputation for rock solid stability and a powerful feature set.
I would also STRONGLY recommend you search around on
http://www.dslreports.com/ before you buy any home gateway. Especially D-Link or Netgear. Those companies put out plenty of great stuff, and plenty of utter garbage. It varies widely from one product line (and sometimes different versions of the same model number) to another. If you search the forums on DSLReports (under Equipment Support in the branded forums) for the model numbers and don't find a bunch of people complaining it might be a good sign (or a new product). But if you find a bunch of people with trouble... Stay away!
The problems you are having with your D-Link are VERY common to the cheaper routers. They just aren't designed for anything approaching reliability (and sometimes I wonder if they were designed at all)! From someone who has a pile of $60-$80 Netgear, D-Link, and Linksys routers sitting in his closet (all of which I've purchased to solve the problems you're having and failed) ... Do your homework BEFORE you buy.