  We study the complex symplectic structure of the quiver varieties
corresponding to the moduli spaces of SU(2) instantons on both commutative and
non-commutative R^4. We identify global Darboux coordinates and quadratic
Hamiltonians on classical phase spaces for which these quiver varieties are
natural completions. We also show that the group of non-commutative
symplectomorphisms of the corresponding path algebra acts transitively on the
moduli spaces of non-commutative instantons.
  This paper should be viewed as a step towards extending known results for
Calogero-Moser spaces to the instanton moduli spaces.
