I was also having dismal performance with the Dell 1540 Wireless card until I disabled all the bands I could that I am not using in my house/business. My router is has 802.11n capabilities. With A/B/G/N all enabled I could only get about 5MBps transferring files wirelessly .on a gigabit backbone.
I disabled B/G and had already applied the other settings mentioned above. Now I am getting 17-19MBps.
There really does need to be better "optimization" documentation for Dell business systems. You may assume all IT people should know the ins and outs of some of these settings but with so many vendors out there and so many settings, it's difficult to know which settings should be changed on which cards. I would love it if wireless would truly get to the "auto-negotiation" level of a wired network. If you don't see A/B/G as part of the connected access point, turn it off... or at least pop up a message asking the user if it should be turned off and why you're asking.