[JimsCAL]

As you guessed, a Wifi antenna will get you more range, but you will be limited to unsecured networks. I agree with Gary that a 3G or 4G cellular broadband card is probably the best solution. Most providers have such a service. Here's a pretty inexpensive one recently announced by Net Zero.

Broadband Internet | 4G Mobile Broadband | DSL | Dial-Up | NetZero