Problem with Wireless card ; XPS M1530 ; plzz help...!!

I am using Dell XPS M1530.

the wireless card installed is Intel(R) PRO/Wireless 3945ABG.

I am using BSNLbroadband with a max bandwith provided 2mbps.

Now my problem is,
When I connect to the net via WiFi, I get a speed of 250-350 kbps only.
But,
When I connect via ethernet cable, it increases to around 2mbps.

Why does the speed decrease when i connect vis WiFi??

The network strength is excellent still speed same.

Now when I connect my Lenovo laptop(Similar wireless card, OS- Windows XP) to the same network I get an excellent speed of around 2mbps, even if the network is poor.

Is something wrong with my Dell laptop?

Now if I upgrade my Wireless card to Intel(R) 4965AGN wireless-N mini card, will the speed increase??

What is the difference those two Wirelesscards?

Thanks alot....

What else can I do to increase the speed via WiFi?

Please reply...