There has been a lot of HTPC hating going around lately, so I figured some alternative perspective was required :)

Motherboard: DG45ID ($114) - Intel might seem like a strange pick for both graphics and a motherboard, but despite some initial hiccups (early bios revs disabled HWA) I have been very pleased with the stability and performance of the board and IGP.  I initially selected this board based on reviews where it out-scored both nVidia and ATI IGP based solutions (the 780G doesn't do so well with AM2 CPU because of memory bandwidth) in picture quality tests, and because of its excellent feature set (8 channel LPCM over HDMI, HWA, legacy free design).  I just picked up a 780G board that will be paired with a X4 so some comparisons will be made, but for now I can say that the G45HD delivers excellent PQ (connected to a 42" 1080p plasma).  The only really complaint I have with the system is that it's very picky about the memory that you give it (1.8v 800MHz DDR); and doesn't offer a real clear CMOS function (but that's a different story ;)).

CPU: E5200 ($74) -  It can do h.264 decoding in SW if asked (just don't ask it to do anything else at the same time), but with the G45HD doing all the heavy lifting not much is needed in the CPU department so going for a cheap, cool, and quiet CPU has worked out really well.

PSU: Earthwatts 380 ($35) - There are cheaper PSUs (even some with active PFC), but I like the 80 Plus Earthwatts.  Higher effciency reduces waste heat, and earns you some green cred, so why not.  This build won't come close to needing 380 watts, so a lower wattage PSU could be swapped in.

RAM: 2GB 1.8v 800MHz DDR2 ($24) -  I tried to make some OCZ ram that was in the closet, and it didn't take (needed 2.0v).  I had some ADATA chips in my dev box that worked so I went with that.  Vista needs 2GB, so don't get less, 4GB is not much more so it might not hurt to pick up the extra 2GB.

HD: ($35) - I actually used an old Hitachi 160GB SATA disk that was laying around, so I save some money there.  The cheapest SATA drives that I could find were some 80GB ones for $35, which frankly seems obscene when a 1TG goes for $95. 

CASE ($?) - I reused my first HTPC case (a micro-ATX Coolermaster), which had been sitting in the closet for 4 years (didn't like it that it didn't have a VFD soon after I bought it).  A decent HTPC case will run $100 (I like the nMedia ones), but state of the working can be had for $15 so I'm leaving this one up to you.  Cases are the only part of a PC that last forever, so I prefer to spend a little more and get something that looks nice and won't slice me up while I play.

Remote ($7) -  I picked up a HVR-2250 not long before I started planning this build, the MCE bundle (includes remote and IR receiver) was only $7 more than the bare card, so I went for it.

Build Cost:  $289 + Case

Of course this doesn't include the software (OS, etc) required to make it all work.  Vista isn't cheap (wait for Windows 7), but all the necessary codecs will be included in the BR drive bundle (do your homework, so you don't get a crippled version of PDVD); or you can also get most anything from the MPC project on SourceForge.  I have a LG GGC-H20L in my main HTPC (haven't picked one up for the extender yet; it's kind of a hassle to do the ISO->Virtual Clone drive thing, but it works :)) which @$119 is pricey for a BR drive, but as an HD-DVD adopter the hybrid nature was nice to have.

This is the quietest PC that I have ever owned, laptops included; it has one case fan, the CPU fan, and the PSU fan.  I don't have a way to objectively measure how much noise it makes, but I can say that the only thing that can be heard when the TV is off is the hard drive, and only when it's under heavy load (which is almost never), a more modern, laptop, or SSD (maybe someday :)) drive would resolve that.