I use the Sony GC83 GPRS/EDGE card and it works pretty well in the car. IT looks like Cingulars unlimited plan it $24 a month. I have an employee plan with unlimited so I'm not sure the difference in price. If you sign up for the ulimited plan I don't think there is anything extra (Though I wouldn't know because of the employee plan...)
I used it in Devalls Bluff, AR and I got about 10K a sec, with 2 bars.
I haven't tried it with any edonkey style programs but Bittorrents worked fine. You can also get an external antenna for it which would make your connection that much better.