htpc is the same as building any other pc...and can just be as good in all areas, because it's a pc
the one thing you have to concern yourself more or absolutely most with, is the noise level of the unit.
you can either try for a build that is fan less and only uses heat sinks (though fans will still be somewhere in the case) - fanless means no fan on the cpu or gpu.
And if you want to buy a standard pc case or a specifically made for htpc case.
many others too, I believe, Zalman makes some too