As said above - make sure you back up before you jailbreak though, makes it easier to restore if it goes wrong etc, usually best to do it with a full battery aswell as if it cuts out half way you've got to go through the hassle or losing all your contacts/apps etc! I'd personally update to 4.1 if you havnt already done so and then jailbreak with limera1n. limera1n/greenpois0n both use the same exploit so either should be good.