Hi there,
well, first off, if you hold a CPL/IR (single-engine) then you can't legal fly an A320 because you'd need a multi-engine rating first. But if you already hold a multi-engine rating, you're fine of course! Irrespective of the number of multi hours you have (the more the better of course) there is actual not that much of a difference between flying a piston multi-engine or a jet (A320), besides the inertia moment (which was already mentioned above).
I personally had the opportunity of flying an A330 full-motion sim (without any previous jet experience) and I must say I was surprised about the easy handling characteristics of this heavy aircraft. I had the impression that it was actually easier to fly than a multi-engine piston (piper seminole or Beech Duchess). The main difference which I noticed was that you have to be well ahead of the aircraft since things are happing much faster, due to the higher airspeed and that if you aren't established on the LOC early enough, then you have no chance of getting it fixed, because of the high inertia momentum of the aircraft.