  In this paper, we describe the architecture and performance of the GraCCA
system, a Graphic-Card Cluster for Astrophysics simulations. It consists of 16
nodes, with each node equipped with 2 modern graphic cards, the NVIDIA GeForce
8800 GTX. This computing cluster provides a theoretical performance of 16.2
TFLOPS. To demonstrate its performance in astrophysics computation, we have
implemented a parallel direct N-body simulation program with shared time-step
algorithm in this system. Our system achieves a measured performance of 7.1
TFLOPS and a parallel efficiency of 90% for simulating a globular cluster of
1024K particles. In comparing with the GRAPE-6A cluster at RIT (Rochester
Institute of Technology), the GraCCA system achieves a more than twice higher
measured speed and an even higher performance-per-dollar ratio. Moreover, our
system can handle up to 320M particles and can serve as a general-purpose
computing cluster for a wide range of astrophysics problems.
