Voltage cells

Having a bit of trouble understanding the basics of voltaic cells. Say I have this setup

So when Zn metal cations break off the electrode and enter solution the Zn electrode will be slightly negative and the electrons will flow to the less negative (since copper is less reactive) copper electrode. Why would you need the zinc sulphate and copper sulphate though, wouldn't the same thing happen if both beakers filled with water?

Heres my guess as to the exact sequence of events: The Zn being more reactive than Cu, dissociates to a greater extent than Cu making the Zn electrode a bit more negatively charged than the Cu electrode so electrons flow from the Zn to the Cu. Then since the Cu electrode is slightly negative, Cu+ cations from the CuSO4 oxidize the it and because the electroneutrality of the beaker with the Cu is disturbed, SO4- anions flow to the Zn beaker through the salt bridge. What I don't get is why the Zn+ ions from ZnSO4 don't automatically oxidize the Zn electrode as soon as it becomes negatively charged. Its a tricky concept to visualize but breaking it down into parts I think I see it now. Its all a matter of the dissociation constant of Zn being greater than that of Cu isn't it?

[Edited on 15-8-2010 by cnidocyte]