The card is old enough that it won't allow you to play HDCP material. Also, it won't offload any of the newer HD video codecs like h.264 and VC-1 from the CPU.
So, what does that mean? Your CPU will have to do all of the video decoding for playback through software. For 1080p at high bitrates that will require a pretty fast CPU.
For the HDCP issue, you can get around it using software like AnyDVD. It's probably better to just get a more modern video card for HTPC duties.