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It is widely recognized that security issues will play a crucial role in the majority of future computer and communication systems. Central tools for achieving system security are cryptographic algorithms. For performance as well as for physical security reasons, it is often advantageous to realize cryptographic algorithms in hardware. In order to overcome the well-known drawback of reduced flexibility that is associated with traditional ASIC solutions, this contribution proposes arithmetic architectures which are optimized for modern field programmable gate arrays (FPGAs). The proposed architectures perform modular exponentiation with very long integers. This operation is at the heart of many practical public-key algorithms such as RSA and discrete logarithm schemes. We combine the Montgomery modular multiplication algorithm with a new systolic array design, which is capable of processing a variable number of bits per array cell. The designs are flexible, allowing any choice of operand and modulus. Unlike previous approaches, we systematically implement and compare several variants of our new architecture for different bit lengths. We provide absolute area and timing measures for each architecture. The results allow conclusions about the feasibility and time-space trade-offs of our architecture for implementation on Xilinx XC4000 series FPGAs. As a major practical result we show that it is possible to implement modular exponentiation at secure bit lengths on a single commercially available FPGA.