Check your bootloader first. I bought 8GB iphone on Friday and it had 2.2.1 firmware and 2.30.03 baseband. I thought i was going to need turbo-sim for it. Got home, followed the guide to check bootloader on it and it turned out to be 5.8. 30 minutes (15 minutes to jailbreak it, 5 minutes to check baseband, 5 minutes to downgrade it) later my baseband was at 2.28.00 and I was happily surfing the web with my old Att sim.