Unlike wishy, I assume you are not looking for anything dodgy, just that you have acquired an old HD box and live in a house with a dish and two feeds from it already.
1. You can can use it as it is with no card, but you won't get all the channels and the recording features won't work. But it will confirm that everything works and that you don't need a professional installation.
2. You can subscribe to a Sky package and get whichever channels you like. And yes, you can get a regular, non-HD subscription even with an HD box
Look here for a bit more detail
http://www.digitalspy.co.uk/forums/showthread. php?t=683213