Well I've tried Purevideo for XP and it was nothing great. Just offers some enhancements like colour, and such. Basically, its good for Mpeg2 video only. Didn't notice any performance improvement, but then I was using CoreAVC at the time.
However, on Vista, Cyberlink has this Purevideo thing built in. So if I use Cyberlink's decoder with Media Player Classic, it uses my GPU to give me a performance improvement over CoreAVC. This is variable, as sometimes i get better performance with CoreAVC or sometimes with cyberlink.
The best for picture quality in my opinion is FFdshow, but to use that decoder for high definition on needs a high end CPU like the C2D E6700 or something. FFdshow I think is single threaded and is not GPU accelerated (neither is CoreAVC though).