1. A PC-level computing system capable of real-time graphics rendering and display operations using a graphics hub device, said PC-level computing system comprising: CPU memory space for storing one or more graphics-based applications, a graphics library for generating graphics commands and data (GCAD) during the run-time of the graphics-based application, and a vendor GPU driver for each graphics processing unit (GPU) supported within said PC-level computing system; one or more CPUs for executing an operating system (OS) and running said graphics-based applications in said CPU memory space, building up a 3D scene containing graphics; a display device displaying said pixel-composited images of said 3D scene during the run-time of said graphics-based application; and a multi-mode parallel graphics rendering subsystem (MMPGRS) supporting multiple modes of parallel operation selected from the group consisting of object division, image division, and time division, and said MMPGRS including; (i) a plurality of graphics processing pipelines (GPPLs), each employing at least one said GPU, and collectively supporting a parallel graphics rendering process that employs said multiple modes of parallel operation during run-time operation of said graphics-based application, in order to execute graphic commands and process graphics data and render pixel-composited images of said 3D scene containing graphics for display on said display device during the run-time of said graphics-based application, and wherein each mode of parallel operation includes at least a decomposition stage, a distribution stage and a recomposition stage; (ii) a decomposition module, implemented within said CPU memory space, for supporting the decomposition stage of parallel operation; (iii) a distribution module for supporting the distribution stage of parallel operation; (iv) a recomposition module for supporting the recomposition stage of parallel operation; (v) a graphics hub device (GHD) for interconnecting said CPU memory space with said GPPLs, and implementing said distribution module during the run-time of said graphics-based application; and (vi) an automatic mode controller, responsive to performance data collected during the run-time of said graphics application, for automatically controlling switching into, out of and transitioning between said multiple modes of parallel operation, during the run-time of said graphics-based application; wherein said decomposition module divides up said stream of graphic commands and data (GCAD) according to the mode of parallel operation controlled by said automatic mode controller; wherein said distribution module distributes said stream of graphics commands and data to said GPUs according to the mode of parallel operation controlled by said automatic mode controller; wherein said GPPLs execute said graphics commands using said graphics data, and generate partial pixel data sets associated with frames of images of said 3D scene to be composited; and wherein said recomposition module merges together the partial pixel data sets produced from said GPPLs according to said mode of parallel operation, and produces a final pixel data set for display as a pixel-composited image on said display device.