Don't buy the AT&T version unless it is already SIM unlocked. Their policy is that you have to use the phone on their network for 6 months to get an unlock code from them (for their prepaid phones). Or get a factory unlocked version of the 520/525.
Make sure you have an option to unlock it, either via an ATT account or by shelling out for an unlock code because its very possible that you wont be able to unlock it.
I looked it up and it looks like it has 3G bands of 850/1900/2100 MHz.