Is it possible to connect a Samsung Galaxy S II Phone to Virgin mobile or Boost Mobile.?

[Austrias]

Is it possible to connect a Samsung Galaxy S II Phone to Virgin mobile or Boost Mobile. If yes how like I know the phone has to be unlocked with a good ESN but could some confirm and reassure me before doing something like this I'm planing on buying this expensive phone since I liked it so much but don't want to a two year contract and a high monthly payment.