For a HTPC you really don't need much. A dual core processor will work as will any of the newer I3, I5, I7 processors.
For the Graphics card my ATI X1950 worked but for BluRay you'll want an 8 or 9 series Gforce, a HD 4000 or 5000 series ATI card or a Intel G45b or above.(Make sure it has a HDMI or DVI output.
1-2 gb ram as well.
Bottom line is find an old computer upgrade the video card and memory.