IMO, wireless cards are only acceptable if your carrier has a strong 3G signal. And based on your original post I doubt you will have any level of 3G.
An expensive but possibly viable alternative would be a true T-1 line, or a fractional T-1. A few months ago you could get a full T for well under $400 / mo with a 2 year contract. And there is nothing more reliable and stable than a T-1. Try
www.megapath.com and see the current rates.
As a last resort, check and see if you can get an IDSL line. This is essentially 144 up and down DSL over an ISDN circuit. These are usually $100/mo, +/-.