Software radio telescopes are a new development in radio astronomy. Rather than using expensive dishes, they form distributed sensor networks of tens of thousands of simple receivers. Signals are processed in software instead of custom-built hardware, taking advantage of the flexibility that software solutions offer. In turn, the data rates are high and the processing requirements challenging. GPUs and multi-core processors are promising devices to provide the required processing power. LOFAR1, the largest radio telescope, is a prime example of a software radio telescope.In this paper, we discuss an optimized implementation of the polyphase filter bank used by LOFAR. We compare the following architectures: Intel Core i7, NVIDIA GTX580, ATI HD5870, and MicroGrid[7]. We present a novel way to compute polyphase filters efficiently on GPUs, and also discuss hardware limitations and energy efficiency.