Samsung SGH-X426 / SGH-X427

I have a Samsung SGH-X426 from att. And it got lock so i'm going to change to t-moible. A man in a store said that he would unlock the phone and put a t-mobile sim card instead of a att sim card. WOuld it work? ANd where can you go buy ringtones at? T-mobile or att becuase the phone is att and the sim card is t-moilbe? And he was goin gto charge me 60 dollars! is that a lot and how do people unlock phones? 😳