Any card that supports DirectX9 will support DXVA - the 5200 falls into that catagory.
I was using the Nvidia Decoder with my 5200, even though the 5200 doesnt support Purevideo at all, the de-interlacing of the videostream is done very well.
I just reciently tried the Hauppauge Decoder, since I have a PVR-150, and I must say that im impressed with it too. The de-interlacing is just as good as the Nvidia decoder, maybe slightly worse, (albeit that concensus would be subjective to my eyes only
) and CPU usage is down for me compared to using the nvidia decoder.
I agree that codecs in general are a mess, especially with 4 already installed with MP. Hopefully its something that can be addressed in the near future.