mmm, ok so I would expect some reduction in signal, and it would be worse if you have solid concrete walls instead of wood partition walls, WIFI signals tend to go in a straight line, so looking at your lovely picture it would be going through 2 walls. depending on those walls depends on the strength you'll get.
The only thing I have found is that my laptops connection depended on what mode of wireless I had connected, I have an N laptop and yet if I set my router to N only, it would only connect at G. If I set the type back to mixed, the laptop connected at N, odd!!!! So try manually setting your mode on the router to the mode of your laptop and see if the signal gets better or worse.
I dumped the D-link firmware and installed the custom DD-WRT firmware from this site (http://forums.dlink.com/index.php?topic=13599.0), and my router is vasty better performing now, so if you haven't already, it may be worth looking into that, the install is easy, i just updated it using the firmware update page on the firmware that came with it. so the issues I was seeing did't seem to be with the hardware it seems to be more software based. My router is quicker, more reliable and so much more stable its amazing.