WIFI network issue in dell XPS l501x

I bought dell XPS L501x in 2011. now a days facing issues with WIFI network which slows the wifi speed.

whenever connecting directly to LAN, it works well with good speed but switching to WiFI network , its speed goes down.  this laptop run on windows 7 home premium (64 byte operating sys). updated drivers many times but did not get any difference. 

If i go to upgrade the Intel Centrino Wireless-N 1000 card in my 2011 Dell XPS 15 to dual band network adapter.

will it work on laptop ?

if yes then please let me know which dual band card i need to buy.?

Thanks.