If i buy an unlocked cell phone from att or tmobile can i use it with boosts?

No you cannot, just as buwish said, AT&T and T-Mobile use GSM networks, and Boost uses the Sprint network, which is CDMA. I do believe you can bring an unlocked phone from US Cellular, Verizon, or Sprint and use it, but it would be mainly up to Boost if they want to accept it.
 

That's what I said...