Do I need the PureVideo Decoder to play movies on the GeForce 8600 and 8500?

No. NVIDIA’s video processing architecture no longer requires the PureVideo Decoder, just a compatible third-party software movie player. The PureVideo HD technology is available through any player that uses the Microsoft DXVA 2.0 interface, including the Vista Windows Media Player with built-in MPEG2 decoder. To play Blu-ray or HD DVD movies, an AACS compliant movie player from companies like CyberLink and InterVideo is also required. Specific versions of those players made for the GeForce 8600 and 8500’s second generation video processor should be available.