I wouldn't worrry about the Extra expense of making an absolutly Silent PC.
With carefull selection of case, Power supply, fan selection, you can assemble
a PC that while not silent will be Very Quiet.
Iam running a 450watt antec truepower PS with a 120mm Fan in a sonata 2 case
with 2 Antec 120MM quite fans set to medium speed ~1150 RPM with a
normally cooled ATI X850XT and a TT quiet CPU Cooler ~19db. And you can
barely hear it running even without the TV on, With the TV going or when listening to music you would be hard pressed to know its running at all and temps have never been a issue with a AMD X2 4200+ the highest Ive ever seen
is 45 C.
While passive cooling is the ideal solution for a HTPC, It is certainly not an absolute Necessity, just pay attention to What you are putting together,
P.S. Fan controllers are Cheap and wonderful little items that give you the power to cut back the noise and if you want to do some heavy duty gaming to crank it up and let her rip