Last I heard the system specs would be similar to The Sims 2, though that's probably incredibly old now. If it's going to be all procedural, it wont be very big at all, because nothing is pre-stored, the computer generates it all according to algorithms. It's pretty rare to see a game bigger than about 4GB, even if it is Spore. It will probably be quite CPU heavy, but nothing that a decent-spec system these days can handle, but generally it should probably be expected to run on 1GHz processors with 512MB of RAM (even Oblivion runs on that, and that's the closest parallel I can think of). That's pretty standard for most games on XP, but of course the more the better.