Flashing is suitable for CDMA mobile models only.If you have GSM locked phone means,No need to flash your phone.Just unlocking is enough.
You can unlock your phone with a help of your service provider or else try to buy an unlocking service from any one of the online unlocking service provider like Mobile-Unlocker.com
After unlocking you can't use boost service.Because it's CDMA carrier.Your phone only supports GSM carriers only.