  To any finite group G in SL_2(C), and each `t' in the center of the group
algebra of G, we associate a category, Coh_t. It is defined as a suitable
quotient of the category of graded modules over (a graded version of) the
deformed preprojective algebra introduced by Crawley-Boevey and Holland. The
category Coh_t should be thought of as the category of coherent sheaves on a
`noncommutative projective 2-space', equipped with a framing at the line at
infinity. Our first result establishes an isomorphism between the moduli space
of torsion free objects of Coh_t and the Nakajima quiver variety arising from G
via the McKay correspodence.
  We apply the above isomorphism to deduce generalized Crawley-Boevey & Holland
conjecture, saying that the moduli space of `rank 1' projective modules over
the deformed preprojective algebra is isomorphic to a particular quiver
variety. This reduces, for G=1, to the recently obtained parametrisation of the
isomorphism classes of right ideals in the first Weyl algebra, A_1, by points
of the Calogero-Moser space, due to Cannings-Holland and Berest-Wilson. Our
approach is algebraic and is based on a monadic description of torsion free
sheaves on the noncommutative projective 2-space. It is totally different from
the one used by Berest-Wilson, involving $\tau$-functions.
