h.264 is a video standard like MPEG 2. x264 is a piece of software that creates h.264 video, there are many other software and hardware encoders of h.264 video. ATI 3000 and up and Nvidia 8000 and up can decode h.264 video through DXVA. I don't know the details but a video *properly* encoded with x264 will play back on a DXVA system and be accelerated by the video card. Of the x264 encoded videos I've encountered, the vast majority are hardware accelerated by DXVA on my ATI 3450. I use MediaPlayerClassic HomeCinema edition.
here is the url:
mpc-hc.sourceforge.net/DXVASupport.html
BTW nice setup, looking to do that later in the summer, hopefully with a sweet SSD.