There is no x64 bluray support to my knowledge. Also the best decoders (i.e. codecs) are all 32-bit. E.g. coreAVC for your x264 MKV's gives the best PQ and the lowest CPU use but is 32-bit only. Ffdshow, the best codec for anything else (xvid, etc, is both 32 and 64 bit but works much better under 32 bit. Audio codecs for aac, m4a, flac etc also work well under 32 bit and hardly under x64.
In addition getting 3rd party filters like CoreAVC, ffdshow etc to work on windows media center is almost impossible under win7 and every new version disables more of them. Extensive hacking is required to get it to work.
On the other hand Vista 32-bit works flawlessly as an HTPC platform (which means no other software installed, just the OS, a few codecs and some other video applications e.g. a bluray player like Arcsoft media theater). I also tend to disable auto updates so I can control the configuration and keep it stable.
So, for a trouble free Media Center PC, I definitely recommend a 32 bit build, even though I am a huge fan of Windows 7 and prefer 64-bit OS's generally. They're just not suited to this. I use Win 7 x64 on my workstation and Vista 32 on the HTPC - I'll upgrade it Win 7 in something like 18 months when all the bugs have been worked out and the hacks automated.