I've completed XCOM today and I've enjoyed the experience a lot. It took me quite a while to finish it, but I managed to finish it in normal mode.
I'm now looking at the downloadable content packs. Are these content packs bolt-ons to the main game, or are they totally separate. What I mean is, do I have to start the game and base again, or are these packs totally separate from the main thread of the game?