At this point, the best-case scenario would be for Crimea to secede, whether uniting with Russia or becoming an independent nation. Since the bulk of the Crimean population seems to be in favor of joining Russia, I'd be OK with this, and I imagine the EU might... if that is ALL Putin gets. At minimum, he would have to recognize the new government in Kiev, if not actually turn over Yanukovych.
I don't see the EU (or America) allowing Putin to overrun the Ukraine. We're already bringing out the big guns diplomatically - kicking Russia out of the G8 is pretty severe. I fear that if Putin goes any further, there will be bloodshed. At minimum, there will be fighting between Russian and Ukrainian troops. Frankly, if it does, I think we should abandon diplomacy and go straight for the military option.