I just bought an Iphone 4g from ebay, and when i got it, it's on "itunes logo", so just wondering if i could just restore it on my itunes? then i can use it? or it's better not to update it so i can jailbreak the iphone? i'm not sure what kind of firmware, but what should i do? any advice?

and also, it has micro sim, how can i use my number if ever i unlocked the iphone? just get a new one at at&t ?