I tried searching a lot, but couldnt find a satisfactory confirmation.

I am looking for your help on unlocking my iPhone 4, bb 3.10.01 . I came across news that an unlock(SAM) is available for the same. But I need confirmation on that. Can you please help me with the following queries?

(1) I am currently using the Phone unlocked with Gevey SIM. THe phone is locked on AT&T - USA..Would you recommend SAM?
(2) I am currently using 4.2.1(8C148). What is the highest version I can move to and still have the unlock?
(3) Can you please guide me the steps I should be taking for the upgrade, jailbreak and unlock?

Your help will be really appreciated