GPU Option Highest Available CUDA Enabled GPU for Desktop System.

Which latest and high end CUDA Enabled GPU would suit my Desktop(OptiPlex 780), for 3D Image Processing Applications??

When I browse through the available products for Desktops(Considering Tesla series only for Workstations and not for Desktops) ,What important parameters that i need to consider ( CUDA Cores? or memorySpecs ? or what specifically??)