" I first ran the CD like your supposed to, the router configured and set up and everything set up with the LAN computer down stairs" The router even came with a CD? Rule of thumb, never use the CD. Not with modems, not with routers, not with printers. All they add is junk. (Nothing personal Dlink, just true in general)
Now lets have a talk on transfer rates, because there is so much confusion on this there is a whole sticky devoted to it. First mistake people make is MB vs Mb. There is 8 bits to a byte, so the 300Mb is Megabit, small B. Thats roughly 37.5 Mega bytes per second. I say roughly because its not direct translation, in both accurcy and process. I won't go into the why, but understand that the 8:1 isnt accurate. Now, that 300Mb is only between two computers, and only when your using N only, 5.8ghz and usually with no security enabled. Also, even if you have a connection rate of 300Mb, that doesn't mean you can transfer information that fast. Thats the FASTEST data can be transfered between the two points, not the adverage. Also you have process times(to render the data to what ever transfer protocal your using) QOS on the router, complex wireless phenonmon..and so on. Think of the 300Mb as the milage you get on your car. A ton of things have to go perfect, and even then your probably not getting it in real life. But that isn't to say it still isn't pretty damn fast.
Like I said before though, thats transfer rates inside your LAN, or from PC to PC. Internet to PC is a completly different story. That speed is limied by some of the same things that limit PC to PC, but a ton of other stuff too. The QoS cap imposed by your ISP mostly. Also the site your using to test, AV software your PC, time of day, distance from telco(DSL) amount of people on node, and so on. But your doing a comparsion, so I won't go into all that.
As for your connection issue, I need to know more about your configuration. First, I would say go wired to the router, and check your speeds there. That will let us know wether to focus on the WAN to LAN or wireless issues from PC to LAN. If wired is fast and wireless is slow, whats your signal strength on the Dlink compared to the belkin. That will tell us to look at either 1st or 2nd layer (ingress,radio, channel, etc) or 3-5th (security, QoS, firewall, etc). I love a good chalange, just let us know.