First of all, what is the speed of your connection to the internet thru your ISP? Most people are bottlenecked there since their wifi card is usually (not always) faster then their connection to the internet.
There are 3 common wifi standards, 802.11b, 802.11g and 802.11n. They're respective speeds are 11Mbs, 54Mbs & 300Mbs. Which of these do you presently have in your laptop?
Unless your laptop is really old, it most likely has g or n wifi capability. If your laptop has a g card now AND your internet connection speed is faster than 54Mbs then you would see an improvement by upgrading to an n card.
If you want to upgrade your laptop's wifi capability, you could contact Dell to ask if they have a card to upgrade the built-in wifi in your laptop. I did that for the Dell laptop I'm typing this post on.
An easier way to upgrade is to purchase a USB wifi adapter. In that case, you would disable the built-in wifi, plug the adapter into an available USB port and install the drivers
that come with the adapter.
If you transfer large files between your laptop and other computers on your network, a faster wifi card will make a big difference but unless you have an exceptionally fast internet connection you probably won't gain any download speed.