You're probably better off looking at some "message boards" which are dedicated to aspects of wireless telephony. As someone else recommended try out. Also check out . There are people in both these forums who know (or at least think they know) about things wireless. Be warned that you will find some people (myself included) to be very opinionated.
A SIM is a SIM basically. The only difference between a SIM used for prepaid GoPhone and monthly billed is just the billing arrangement. The SIMs are not different. Only the billing arrangement is different. The only problem you might have is if you had an original AT&T Wireless GSM phone.