$400 is way too much for what you propose considering that many sites offer refurbished systems for much lower. Tigerdirect.com offers dual core refurbished systems for under $200 right now (I saw one for under $150 today actually) plus tax and shipping. They may not be tiny cases, but are much cheaper and are full systems. Keep in mind that they also have barebones kits as low as $250 that have triple core and on special occasions even quads for under $300 that can be easily put together. If you feel under qualified, do a few youtube searches for assembly and you should have enough knowledge for a barebones kit.

I am not recommending this machine for crunching, but it's the closest thing I know of to your size, price, and power preferences:

http://www.newegg.com/Product/Product.aspx?Item=N82E16856107080

This model (which Jetway is calling a "barebone plus") comes with a hard drive on which the operating system is already installed. The user would have to install a memory card, and would have to install BOINC.

I have another version of this same machine -- the "barebone" with no plus. I installed a hard drive and 2 GB memory, and the OS (for which I chose Ubuntu). It's been running 24/7 for nearly a year, mostly crunching WCG (although that's not what I bought it for). It's very reliable but SLOW. Since it has a dual-core CPU with hyperthreading, it crunches 4 WUs at a time. So it turns out 4 HCC results every 8 hrs. It has a 60 W power supply, which is great plenty for the Atom/Ion. The wifi is decent.

Zotac and Foxconn make similar machines, but I don't think any of them come with an operating system.

This probably isn't helpful info, but I thought I'd mention it just in case.

Well if barebones scare your target audience, it would still be cheaper for them to purchase those and pay a tech for their "hour of labor" to assemble them. Installing Linux is actually pretty simple and fast and Windows really isn't that rough either. So if they are too lazy to watch a few youtube videos to see how easy they are to assemble, then the average $65 labor of most techs would still be cheaper. Yes as I did state, those cases were larger then you were looking, but then again you are asking for cases that typically are a lot more expensive anyways. I doubt you will find a supplier to dish out what you suggest and still have a market willing to buy them. Those of us who normally would look at it have the skill to build bigger better faster and sometimes cheaper systems. But, if this is some business plan of yours, I certainly hope it pans out. I just don't have much faith in what you propose.

Edit: I noticed you mentioned better to find used core 2 duo's...did you look at the Tigerdirect off lease dual cores that were in that price range. Again, they aren't min/micro cases. Then again, they really don't sell many systems in micro cases anyways. I doubt you would find many used systems that small. Most businesses stick with standard size towers.

Exactly. Most high end CPU's don't care much for the small cases because of poor air flow and most people don't like loud fans. So, the result is typically Atom processors. I'm not saying it can't be done, just not likely many will do it. I always wanted to replace my wireless router with a PC that had a wifi card designed to be an access point and internet connection sharing, but not many cards out there for this niche. One less device to power and it could crunch while routing. Not to mention just slightly larger and could also be a home theatre PC.