  We study the Casimir interaction in the plane-sphere geometry in the
classical limit of high temperatures. In this limit, the finite conductivity of
the metallic plates needs to be taken into account. For the Drude model, the
classical Casimir interaction is nevertheless found to be independent of the
conductivity so that it can be described by a single universal function
depending only on the aspect ratio $x=L/R$ where $L$ is the interplate distance
and $R$ the sphere radius. This universal function differs from the one found
for perfect reflectors and is in principle amenable to experimental tests. The
asymptotic approach of the exact result to the Proximity Force Approximation
appears to be well fitted by polynomial expansions in $\ln x$.
