When you say a reboot of the card, exactly what do you mean. Does the system reboot? In that case, you're looking at some hardware problem.
did some checking... that CPU really does not handle HD (at least h264) decoding all that well.. so the options you have is to get an a good GPU and use a program that would let the GPU do the decoding (like XMBC or Media Player Classic/Core Codec) or upgrade your CPU to a Core 2 Duo or better
interested in a used 4670... she def handles HD content and has HDMI audio/video out via the second dvi