Most people will tell you to search. Since you're new, I figured I'd spare the BS and actually answer you.
I use a wireless NIC card in my setup. Pretty much because I don't want to spend extra money on a cell phone or GPRS modem equivalent setup. The wireless Inet card that I have is nice, but I'm not a wardriver or anything.
I pretty much use it to sync files in my garage, between my carputer and server in the house. It's also nice to connect to my own internet and upload logs to www.whereismygps.com and for GPSecure. Other than that, I have periodically parked outside a coffee shop that offers free wireless internet to pull down my emails.
Overall, the GPRS modem or cell phone setup would be ideal. It's a dedicated modem connected directly to your carputer. If you do opt to try the wireless 802.* card method, I suggest you mount the antenna in the window, or get one of the external wireless antenna mounts. That will (obviously) help with your signal reception.