Wait until your next new computer(s).. If you buy new ones now, you will get Windows 7 with a cheap upgrade path to Windows 8, but as Pine Man says, if they are OK for you now, leave things as they are.
Pine Man hits one nail on the head regarding security, Windows 7 is more secure than earlier versions. Your XP machines may not have the capability of running Windows 7 but the Vista machine will. Personally I would upgrade this one to Windows 7; you will find it will be more secure and run faster. Windows 8 has not been designed for simple desktop machines, to get the best from it a touch screen monitor will be needed.