I'm sure The Sims would look great if it was powered by Frostbite 2 but I doubt that would be the case. The engine would be too demanding for the people that play The Sims. I can't wait to see it with Command & Conquer: Generals 2 though.
As for Dead Space 3, I'm really looking forward to the improved animations. They were great in Dead Space 2 but they'll be even better with Frostbite 2.
I doubt it, it's only as demanding as the content you push in it. The only reason Sims 4 and Frostbite 2 might not be a good fit is there's undoubtedly a lot of custom code in the Sims 3 engine that I would imagine them reusing, and it'd be hard to do if they switched engines entirely.
I haven't finished DS1 since I ran out of ammo so I've barely started 2, but I own them and this years gaming selection is pitiful so we'll see. I got major catchup work to do on last years games so hard to say.