What is nut width?
 

I'm looking at RAN guitars custom website just to get an idea of how much a custom would make but i don't know much of the terminology. I know roughly what nut width is but what does it determine on the playability of a neck? if anyone could tell me what the nut width of a Gibson SG(faded is the one i most recently played) and the width of an Ibanez Rgt42dx. i like the neck of the Ibanez so what nut width would be best?

Standard 6 string Floyd Rose style locking nuts are 42mm wide, this will apply to the Ibanez.

what about a string thru flying v shaped guitar?

It might be slightly narrower. You may be able to find the specs online somewhere. Google :)