The sun and its remaining planets likely won't collide, but their orbits will be so strongly perturbed that the resulting galaxy won't look anything like our current spiral. With the spiral galaxy, orbits are fairly orderly - stars around us are more or less moving with us in our journey around the galaxy. After the "collision", things will be different, with much more randomness in their orbits around the galactic center. Right now we are in a low-density area of the galaxy. After collision, who knows - we could end up going through some high density areas where planets are often removed from their orbits around stars.
However, if we manage to survive that long, that means we will have become a space-faring society, and will no longer be tied to any particular planet. In fact, Earth will have long been destroyed, so long before such a collision occurs we'll be living in the vicinity of some other rocky materials.
