All USB connections (mini/micro/etc) are 5V. The USB connection from a computer is only capable of 500mA though (min spec is 100mA, I think). The stock charger does 1A and I've seen other chargers do anywhere from 500mA to 2A.
Does it matter? Yes, kinda. The way the phone charges, it will only draw as much power as it needs. Will a 5V/2A charger charge faster than a 5V/1A? Probably not.