What do you think of this argument for God?

[Amarok]

I heard an argument like this:

1. God is the greatest possible being.

2. God is a necessary being, which means that God exists in every possible world (If God exists). 

3. If God exists in one possible world, God must logically exist in every possible world. 

4. Since God is the greatest possible being, it follows that every aspect of God (being possible) exists in some possible world.

5. Therefore, God exists (in all possible worlds, including ours).

I actually just structured the premises this way myself but is the same idea as an argument I heard before. 

What do you think of it?

1. The key point here is him existing in any possible world .And the idea there can be a greatest being . Or the assertion that a god is in anyway necessary. Oh and you can revise this argument to show god doesn't exist. Overall the seems like a rehash of Alvin the goat man and has been man handle by far better then me. But interesting none the less.