RE: Dell Wireless 1397 WLAN Mini-Card
The Dell 1397 Mini-Card is a b/g rated wireless adapter. However, your router is N rated, thus the N600 name. G ratings max out at 54mb download, however, I had the same issue with performance. I have 65MB down when hardlined and 21 running wirelessly.
Your best bet is to buy a Dell DW1515N rated wireless mini-card and swap out your current one. I just bought one on Amazon and it cost me only $25. Here's a link to that item if you're interested.
It's very simple and can be done in about 2 minutes. Here's a tutorial video to help.