This could be related to any numbers of factors you have failed to mention, namely interference, updates needed for the router, placement of the router for optiumu signal strength, etc.

Updte the drivers for both cards, remove encryption and filtering  and see what improvements you get.  If none, this is as good as it gets.  Of course, if you see no improvement, re-enable WPA at least.

are you talking download speeds from the web or transfer speeds between pc's on your wlan?  If those are wireless download speeds from the web you may need some tweaking.

Your internal LAN speeds will never surpass these numbers:

 

Wired:  100 mbps

Wireless:  54mbps

 

Now, if you can get 50% of the posted wireless speeds, consider yourself lucky.  Wireless was built for convenience and not day to day opeartions.  Do I think the environment is "too noisy"? Possibly so, but only the testing of another router can confirm this.

so, your wired ethernet seems to be ok including internet download speeds since you're getting near your provisioned max.  Your wlan internet speeds could be better and may require some tweaking... you might consider running a tweak test at dslreports, or download drtcp and see what your rwin is set to for the wlan adapter (what is the rwin now?).  As mentioned by jmwills, you may also have some interference on the wlan due to cordless phones and other stuff.  If tweaking does not help, try changing channels to 1 or 11 (what channel is it set to now?)?   wrt54g only has b or g or mixed b+g settings (no n)... which one are you using?  disable encryption temporarily as suggested to rule it out, and then try using wpa2 personal.  If you have no n wlan devices, then on the dlink just use g.  Any other wireless pc's on the wlan when you are running the speed tests?  the dlink di-655 10/100/1000 ethernet is helpful for file transfer between gigabit devices....but will not speed up transfers to 10/100 devices.