Aircard for Dell Latitude C640

I have a Dell Latitude C640 laptop and I want to know if I can buy an Aircard to access the internet.  I already have a wireless card installed in the laptop but I want an aircard so I can have internet access anywhere, not just where a wireless network is available.  A friend of mine has one and he loves it but he has a newer Compac laptop so this might not even be available to me for my Dell Latitude.  He says Verizon charges him a monthly rate for unlimited internet access, just like the monthly rate you would pay for internet service at your house.

 

Can I buy a wireless card for my laptop?  If so, what brand/type will work for me?  Any information would be greatly appreciated.  Thanks in advance!  Kristin