I'd not recommend you use an internal wireless adapter unless it is a laptop. Laptops usually have the antenna routed either around the bottom plastics or around the LCD, but desktop systems using internal cards rely on a small POS antenna that sticks out the back and if your case is stored inside a desk or close to a wall, this causes interference problems in many cases. The USB adapters are best for desktops IMPO, and PCMCIA adapters work best for laptops, though systems that route the antenna around the LCD are almost as good with a mini-pci.
I'd also not recommend D-Link, their support is crap, their units are junk and they have compatibility issues, especially with their + series which is not fully compliant. I'd recommend Linksys or Orinoco, but whatever you buy, do try to get the same manufacturer cards, Liquidlen is correct about that part, using same manufacturer hardware will ensure the best results, especially if you plan to use range entending or speed boosting tech(these things are usually not universal standards)
A few other tips, don't locate your router in a closet, that's asking for interference problems. Get rid of your 2.4GHz phones. Update the firmware after setting up the router(its probably a couple of versions old) This is especially important if you are using one brand router and another brand cards. most important...RTFM!