HTPC Under 400

Oh and I already have the keyboard, mouse, monitor, speakers and os.

AMD A6-3500 Llano 2.1GHz (2.4GHz Max Turbo) Socket FM1 65W Triple-Core Desktop APU with DirectX 11 Graphic AMD Radeon HD 6530D AD3500OJGXBOX
bundled with:
1x GIGABYTE GA-A75M-D2H FM1 AMD A75 (Hudson D3) SATA
http://www.newegg.com/Product/ComboDealDetails.aspx?Ite...

SAMSUNG Black 12X BD-ROM
http://www.newegg.com/Product/Product.aspx?Item=N82E168...

($65) CPU: AMD A4-3300 Llano 2.5GHz Dual Core
http://www.newegg.com/Product/Product.aspx?Item=N82E168...

($70) Motherboard: ASRock A75M-HVS FM1 AMD A75
http://www.newegg.com/Product/Product.aspx?Item=N82E168...

($29) RAM: 4GB Mushkin (or whatever, really)
http://www.newegg.com/Product/Product.aspx?Item=N82E168...

($80) Hard drive: Seagate Barracuda 250GB 7200RPM
http://www.newegg.com/Product/Product.aspx?Item=N82E168...

($30) Case: Linkworld HTPC case with PSU (you can pick a tower if you want)
http://www.newegg.com/Product/Product.aspx?Item=N82E168...

($58) BD Drive: Samsung 12X
http://www.newegg.com/Product/Product.aspx?Item=N82E168...

That's $402. The CPU isn't the most powerful chip there is, but it's solid, it's got a great iGPU, and an HTPC doesn't need a lot of power. The mobo has some nice features for the price including HDMI, SATA 6 Gbps (which will be nice if you ever get an SSD), and USB 3.0. The RAM, hard drive, and case are nothing special. If I had a higher budget then the first thing I would do is to get a slightly better case without a built-in PSU but the components have a really low power draw so the 400W PSU with the case will be fine.

It's hard to pick a blu ray drive. BD playback is kind of an infuriating business right now so I'm not really sure how to pick a good drive.

FYI, akopp21's CPU is better but I couldn't get it in under $400. If you're willing to pay an extra $25-$30 then get the triple-core.

Llano seems overpowered for a HTPC (read "runs hot and noisy in tiny enclosures"). I assume you want this thing to be small so it's not overbearing near your TV. I went with Brazos. E-350 on a mini-ITX board. Gigabyte one- GA-E350-M1 or something like that. onboard everything. AMD 6330 graphics. has HDMI, DVI, VGA, 7.1, gigabit, I think it even has USB3 but I'm not certain.

cool, quiet, and capable. And almost certainly cheaper than $400 to build out. but verify in reviews to make sure it can do what you need it to do.

HTPC is all about being just powerful enough for what you need, and no more, because extra power means extra watt usage means extra heat means extra noise and extra noise is the bane of the HTPC. also, a low wattage solution like Brazos means you can keep it on all the time, and potentially keep it on inside a semi enclosed entertainment center without overheating.

The all-in-one motherboard/cpu solutions like the E-350 are capable units but I would only recommend them in an extreme budget situation where one cannot afford at least a Llano. A Llano isn't going to overheat at stock speeds running htpc applications. They're not ultra-low wattage but they're only 65W meaning that fan-noise and heat won't be a problem.

Because of the great iGPU, Llano processors are the most common recommendation I see for low-budget HTPC's.

A raspberry pi will be your cheapest solution. It only has 2x USB ports so you will need to purchase a USB hub to have your USB hard drive, USB Blue ray drive, keyboard, and mouse all plugged in simultaneously.

$35 Raspberry Pi -> http://www.raspberrypi.org
$20 USB hub -> http://www.newegg.com/Product/Product.aspx?Item=N82E168...
$60 Blu-ray Drive -> http://www.newegg.com/Product/Product.aspx?Item=N82E168...
$70 2.5" USB HDD -> http://www.newegg.com/Product/Product.aspx?Item=N82E168...

That's only $185, so you've got some breathing room.

You will have to wait til February as the raspberry pi's are just beginning production now and are not yet available for sale.

Edit: for those that aren't aware, a raspberry pi is a small computer that is designed as a programming learning tool but has many other possible applications. It is the roughly size of an iphone but does not have a silly case hiding the cool innards. It has:

2x USB 2.0
1x HDMI
1x SD Card Reader (OS is installed here, various linux distros available)
1x 35mm Audio Port
1x RCA Video
1x HDMI
1x Micro USB Power
1x Ethernet (on the $35 model, excluded on the $25 model)

It runs on a 700mhz CPU and comes with 128 or 256 mb of memory.

While these specs seem lack luster, consider the price tag and look at the real world tests that have been done so far.

It is designed to offer 1080p video play back with no stutters, and there are videos of quake 3 running quite smoothly on it.

That raspberry pi is pretty cool, but I don't think you can even run XP on 700mhz and 256mb.
The FAQ says it doesn't support Windows.

That's quite right, danrales. All HTPC activities aren't very OS specific, so I don't know why that would be an issue.

It certainly doesn't work for many people needs, but it's a very affordable new option for us all  .