Ukraine is not a full member of NATO, but Western Europe would not sit back and watch Russia actually invade the Crimean peninsula. So, if the Kremlin were to actually launch that assault, they would most certainly suffer casualties and serious losses of equipment and material, as well as economic consequences.
I do not think that the Kremlin should try to overrun Crimea. I think that their invasion of the Ukraine is unlawful and a breach of international law. I think that they should be punished by the international community for their crimes. Russia should leave the country of Ukraine as soon as possible.
As an American, first let me state that I am largely opposed to our foreign policy and think we need to withdraw from many of the countries we're in and need to scale back our military. In Russia's case, I think this is dangerous. Putin is a dictator simply trying to restore the former USSR. If he suceeds, then what?
I think that making a rash decision like that would incur the wrath of Western Powers who have warned Russia to not make any overly agressive moves, I believe overall it would be a strategic mistake to go as far as over running Crimera and I believe the Russian Government is smart enough to realize this.