Hybrid architectures are systems where a high performance general purpose computer is coupled to one or more special purpose devices (SPDs). Such a system can be the optimal choice for several fields of computational science. Configuring the system and finding the optimal mapping of the application tasks onto the hybrid machine often is not straightforward. Performance modeling is a tool to tackle and solve these problems. We have developed a performance model to simulate the behavior of a hybrid architecture consisting of a parallel multiprocessor where some nodes are the host of a GRAPE board. GRAPE is a very high performance SPD used in computational astrophysics. We validate our model on the architecture at our disposal, and show examples of predictions that our model can produce.