There is little reason for Media Center users to upgrade or install Vista x64.  Microsoft has shown that it’s still not ready for primetime with several of their applications not working correctly on x64.  For Media Center users and most consumers in general, installing x86 will serve you a lot better.

Not only Microsoft’s own applications (such as Internet Explorer, which has several issues), but drivers are another huge problem.  With several companies lacking in the x64 driver department, it’s going to take a while for them to really get where driver support should be.

Just install x86, maybe look at x64 sometime next year.  Right now, x64 is just going to give you more headaches that aren’t needed.