hauppage is pretty much the cream of the crop for the home encoders
You can spend big bucks on others if you're so inclined, but hauppage 150 is a great card
the biggest thing you want to look at is having as much stuff as possible passively cooled
also, VLC is fantastic for streaming content from a fileserver/pvr to other parts of the network (unicast, or multicast)
do you have a receiver/surround sound unit existing? if so, just get a sound card with spdif out (optical, coax, whatever)
if you're not gaming, I wouldn't worry about a mobo with SLI, or the 7.1 onboard
look for one that has spdif onboard
for a HTPC you really don't need a lot of processing power, especially if you've got a hardware encoder
currently I'm using a p3 933 all air cooled, and it's almost 100% silent (I can't hear it from 6 inches away with the glass door closed)
The biggest thing to look for(imo) is a mobo/case that will give you the ability to expand as time goes on
here's a
really good article on what to look for in a HTPC
buddy of mine that I work with has also done some kickass
mods for his HTPC (warren liick)