Case: Ahanix D4 Media Center Enclosure ($200ish can't find a store with one in stock) - The case comes with a 350 watt PSU, but I didn't use it in my HTPC; Ebay it to make some money back.  The case is the most important thing to get right when building any PC.  It is the part I hold on to the longest, a good case can last through many upgrades; the config below is the 6th HTPC to sit in it.  For a long time I tried to go budget when selecting a case, and always regretted it.  Get the best case you can afford, getting cut up when you're messing around in the case really sucks.  I still prefer a full ATX case, but as HTPC become mainstream the limited selection of decent micro boards has pretty much gone away.  The selection of cases has really exploded since I bought one, and there's even a decent selection in the mid-range (which if memory serves didn't really exist before) of HTPC cases so there is something for everyone.

One negative for this case is this case is cooling, stock it has two 60mm fans in the back and no mount in the front.  Originally I only added a 60mm-80mm adapter to one of the mounts in the back and it cools OK.  Around the time I replatformed on Vista I modified the case for better cooling.  Cutting a hole in the side and adding a 80mm fan dramatically reduced the case temp.  At the time I was having stability problems with my pc that were later isolated to the video card so this modification probably wasn't essential.  Since then my requirements have changed (quad core, live h.264 scanning, etc); so to be on the safe side I added another hole/fan to the top.  Like before, I'm not sure that it was absolutely necessary, but better cooling directly translates into a longer lifespan for your components.

PSU: Antec Earthwatts 380 ($50) - Like the case, a good PSU will last through several PC upgrade cycles.  Around the time my SST-ST40F died, I found a deal on this guy.  While at the time I would have preferred a PSU with a 120mm fan, the efficiency (where efficiency is indirectly proportional to waste heat generation) was very tempting.  And as it turns out, case temp actually went down after installing it.  The PSU is inaudible, has active PFC, and provides adequate power for my system; I am very happy with it.  So much in fact, that I picked up a second for my recent extender build.  The price of this PSU fluctuates a lot (maybe they all do?) so I'd wait for a deal, I only paid around $30 both times.

Motherboard:  Gigabyte GA-MA78G-DS3HP ($86) - When the 780G chipset came out I knew it would just be a matter of time until one found its way into my HTPC.  I wasn't happy with the selection of v1 boards (mostly micro-ATX and not spec'd for 140 watt CPUs).  This board is laid out very nicely, has a lot of USB and PCI-e, and runs 5-8 degrees cooler than the 570 Ultra board it replaced (could be because a discrete video card isn't required).  I was considering 790GX boards, but couldn't find a feature (besides the slightly higher clock speed of the IGP) that really justified the price difference.  The HWA and PQ are identical (very good) to the 3650 that I was using.

There are many other features that are really nice, but I'm going to specifically call out the internal parallel port connector.  If you use a parallel port based VFD chances are you've snaked the cable out the back of your case to and connected it to the back I/O panel.  Not only is this not visually appealing, the cable (which at least in my case has no shielding) runs right over or next to some of the noisiest (electrically) components in your case.  My last board also had this feature, and I was dreading the potential pain of finding an internal LPT card.  The placement of the connector is even more convenient than before (just a few inches from the display)

CPU: AMD X4 9950+ Black Edition (125 Watt) AM2+ ($163) - Putting a 125 watt quad core in a HTPC that lives in a A/V cabinet may seem like an odd choice (last time I said "If your HTPC is going to sit in your A/V stack I'd stay away from a quad core, heat and noise are the biggest concerns, four cores in a box don't play nice with either."); so call me a hypocrite. 

Because it's multiplier unlocked, I had intended to underclock the CPU to roll my own 9550e, but after running it at stock speed and voltage I haven't needed to back it down at all to reach my heat/noise goals (my TV's fan is louder than the PC) so I left it alone.

HSF (Heatsink and Fan): Scythe Mini Ninja ($36) - After seeing a bunch of reviews, and a sale a few days later (got it for $15AR) I figured it was worth a try.  It was originally used with my X2 in a passive (no fan) configuration.  When I added the 9950 I knew that wasn't going to fly, but adding a fan (note the fan that comes with it is too loud) is easy with the included clips.  Much more effective than the stock HSF that comes with the 9950 (installed it first to get a baseline); highly recommended.

Videocard: ($0) - IGP

RAM: G.SKILL 4GB 1066Mhz DDR2 ($55) - Because I went with an IGP that takes ram from the system, and I wanted at least 2GB of dedicated system ram, getting a 4GB kit was a no-brainer.  Besides DDR2 ram is so cheap, even if I had made a different choice for video 4GB would have probably made it's way in.  Using a AM2+ CPU, the 780G supports 1066Mhz, so I went with that (the extra bandwidth is most important for video).

Hard Drive: Seagate 320 GB ($55) : Seagate 1TB ($100) - Get lots of storage, 1TB drives are the sweet spot right now.

Optical Drive: LG GGC-H20L - Retail ($115) - There are cheaper drives but this one does Blue-Ray, HD-DVD (early adopter), and comes with a full version of PDVD 7.3.

Network Card: Integrated Gigabit ethernet.

TV Tuners: AVerMedia M780 ($90) - Great HW, sketchy drivers.  The latest WHQL drivers BSOD regularly so I use the .19 ones.  There is a beta set (.30 I think) that sort out the BSOD, but I'm happy with what I have so why change?

Hauppauge HVR-2250 ($110)  - I haven't tried to use the NTSC tuners, but I can say this is an excellent dual ATSC capture card.

Hauppaug HD-PVR ($209) -  My feelings are mixed on the device.  On one hand, it's the only way to capture HD cable w/o crippling DRM; on the other I'm on #4 (good thing Hauppauge's support is really good).  

Remote: Logitech Harmony One ($190) - Completely optional, but makes the experience so much nicer.  Very nice remote, fits nicely in the hand and works very well.  Harmony's activity based control makes it easy for the family to use.  Wait for a deal, I got mine for $120.

IR Receiver: MC Remote IR Receiver ($23) - If you get a retail tuner, many come with MC IR Remotes.  I saved some $ on my extender build because I paid $7 extra for the retail 2250 kit.

Keyboard/Mouse: Gyration RF Keyboard and Mouse ($85) - A true anywhere in the room keyboard and mouse solution.  The keyboard is small, so it fits nicely away when I don't want to see it.  The mouse takes some getting used to for in the air use, but it is intuitive and really works.

Total: $1567