HD is more a GPU requirement than a CPU requirement. Any CPU that fits into a mobo with a PCI Express slot should do fine enough. Then get a GPU that is HDCP compliant for Blue-ray and other HD content playback. I would suggest at least an 8000 series card (my wife's C2Duo paired with a 8600GT works great, and there is 0 CPU load on video playback as the GPU does all the heavy lifting). I see no reason why a Pentium 4 (so long as it is an LGA socket) would not be able to hack it unless you had other issues (like less than 2GB of Ram). If it is an old P4 on a PGA socket then you may be out of luck as the buss speeds alone would be too slow to put that much data through.
Whatever card you get, make sure that your power supply is big enough to use it properly.