If you are asking about the 3G listed in your signature, you can jailbreak just about any firmware. I am using 3.1.3 on my old 3G. I also upgraded to 4.0 on that same phone and then ran Redsnow to jailbreak. At the time, there wasn't much available in Cydia for 4.0 so I had downgraded back to 3.1.3 and jailbroke it again. On 3.1.3 I used Spirit to jailbreak.