Do what Dizway said about making sure the SSID is set the same on both devices.
Also, you will want to UPGRADE THE FIRMWARE on your router to the latest (using a wired connection). I noticed a significant increase in signal strength once I did that. It is under Gateways on the D-Link FTP site. I believe the latest is 2.57 b3, but don't quote me on that.
You will probably also want to set up encryption (so nosy neighbors don't sniff your packets and read your mail), and you may also want to set up MAC address control (so those same neighbors don't go buy themselves a wireless NIC and lunch off your cable connection, cutting into your bandwidth). The encryption allegedly costs you a bit of bandwidth (which you probably don't need for sharing a cable connection anyway), and the MAC control costs you nothing.
Both are reasonably easy to set up.
Before setting up encryption or MAC address control, you will want to make sure it works well with no encryption and no MAC control first. Once it is working perfectly, proceed with the other two.
For the encryption, just set the encryption level the same on both sides (I use 128-bit shared key encryption between my D-Link 713P and 3com wireless PC Card), and put in the same keys on both sides. Make sure the same key is enabled on both sides. Once that is done, save the changes, re-boot the router, apply the changes to your client card's software, and hope it connects.

WEP Encryption is supposed to cut your bandwidth (and probably does), but strangely, I noticed that the reliability of my connections was significantly improved AFTER I enabled this feature. Of course, now that I have the "new, improved" Comcast Internet (as opposed to real cablemodem service like I used to have), I don't need too much bandwidth to share this connection anyway.
The MAC control is very easy to use. Just put in the MAC addresses of your wired and wireless client cards into the the appropriate menu in the router, set the router to deny access for both connection and association to any unspecified MAC addresses, check the appropriate boxes to enable your selections, save changes, and re-boot the router. The easiest way to do this is connect all of the computers you want to the router, and copy over the macs. You should probably take out the IPs, so that the router can still assign whatever IP it wants to each wireless client without it messing things up.
Almost forgot. One more piece of advice. If you have a wired NIC (integrated or PCI card) in the PC you want to connect wirelessly, disable it in device manager, or your wireless NIC may pull an IP address, but just mysteriously decide not to transmit ANYTHING (except the IP address). Not even a ping. Took me a few HOURS to figure that one out...
Nack