Sorry, I don’t have the time to do the research for ya, but I can lead you in the right direction.
First, it sounds like you won’t need much in the way of a major system, just enough to get by.
If you don’t want to go past 720p then a 3000+ will be fine but if you want to push the envelope up to 1080p or 1080i then a 3200+ is minimum.
Motherboard, whatever you like really, but a mini is best. And as for case, do a search on Google for “Silent PC Case”. It will turn up all sorts of options.
Minimum memory would be 512MB, but I would go with 1GB because you’ll be less likely to see things swapping in and out of memory that way no skipping or freezing while viewing media.
As for hard drives, just about anything SATA will do and it should be more than 100GB if you plan on keeping a lot of media there. I would try to get over 160 but 200 would be optimal, especially over the long run as stuff starts to pile up.
Video card will probably be the most difficult to choose but minimum should be a 1300 ATI or 6600GT nVidia either with VIVO. Both are optimized for better HD playback. I personally went with the Gigabyte 6600GT VIVO for my HTPC because it doesn’t have a fan.
Other things to consider:
I went with no TV tuner and just use a rented box from my cable company. This means I get HD video for around $5 a month. There are up and downs to this method though.
Get a keyboard and mouse with a really long range (RF).
Don’t get a sound blaster. Find a board with good digital SPDIF (coaxial opposed to optical since optical is just way overpriced and coax is just as good) and good reviews. Great alternatives to sound blasters are M-Audio and Prodigy 7.1 – both of which sample at better rates than SB and are better suited to a HTPC. SB are great for games and that’s about it.
Also, look into www.orb.com, it might do some things you want (I use it and LOVE it). Also, do a search on Google for Meedio Center; their free package is a great starter ( I use their full package btw).