A growing number of applications for 3D graphics and imaging systems require the customized approach to the design of high performance 3D graphics and imaging systems. The progress in hardware support of graphics API functionality gives a real possibility to use a top-down approach in the design of custom systems based on the mixed hardware/software implementation of OpenGL API architecture. Software simulation is relatively cheap and fast way for initial architecture template development and evaluation to acquire the structure and key parameters that can be used later for elaboration in hardware description language level. The project includes the implementation of 3D graphics system architecture model based on OpenGL hardware support simulation. The object-oriented approach has been used for this development of analytic and software behavioral models simulating hardware units in different stages of graphics pipeline. Interactive Imaging System Architecture Composer has been developed for fast run-time architecture reconfiguration, simulation management, data acquisition and visual interpretation. The expandable set of architecture templates uses a reconfigurable shared library of "virtual hardware units" simulating various hardware structures for graphics and image processing. The project realization provides a set of simulation and performance evaluation tools for computer architects to cover the first steps in top-down design process. The graphics/imaging system design can be customized and this technology might be expanded from corporate level to public domain to make it available for engineers willing to build graphics/imaging systems for particular applications.