4.11 or 3.73's and sohc or dohc

[T-Bone]

Hey Guys, Thanks for all of your help recently. I've got a couple more questions for you now.
First, a friend of mine, who is a mechanic, told me I should go with 4.11's. I've noticed that most of the GT's have 3.73's and the Cobra's have 4.11's. How much will the two different gears change low end and high end performance? I want to make the right decision and get the gears that will suit me best. I guess I want to get up and go faster. I don't really care if the top speed is decreased that much. What will the top speed be with both gears, if anybody knows?
Second, the mechanic also told me that he's pretty sure it's a DOHC, by looking at it. I'm not sure, but I fiqured it was SOHC since thats what most or all of the 96' GT's seem to be. I have no owners manual to refer to for the specs. How can I find this out for sure?

Thanks in advance!
T-Bone