It's not cheap at $799, but the Fang Mini PC is one of the most envious SFF HTPCs around.
On the market for a new Home Theater PC (HTPC)? CyberPowerPC just let the Fang Mini PC loose, and it's definite;y worth a look.
At $799, it's not the cheapest option for an under-TV PC, but the hardware inside is small, potent, and (to a certain degree) game-worthy.
Two models are being offered; the $889 Intel Fang Mini (based on the Gigabyte GB-BXi7-4770R Barebone PC) has a Core i7-4770R CPU, with Iris Pro 5200 graphics baked in. The $799 model uses AMD at its core -- an A8-5557M APU with discrete Radeon R9 M275X graphics attached (the APU also has integrated Radeon 8550G graphics). While the Intel model will use less power overall, it can still power dual 4K displays if necessary. Powering 4K is great, but don't expect to do much gaming on the Intel hardware, maybe some Orange Box titles aside.
The AMD model, which is the Gigabyte BRIX GB-BXA8G-8890 Barebone PC with a fresh coat of paint, is less expensive (it always is!) but it's definitely more gaming-friendly. The Radeon R9 M275X graphics part is no Titan, but it should be able to handle Medium settings at 1080p with most titles.
Both models come with the following: 8 GB of DDR3 memory, a 1 TB 2.5-inch hard drive, four USB 3.0 ports (two front, two rear), Ethernet, DisplayPort, HDMI, and audio out. There's also an mSATA option, allowing for a small SSD to be paired with the large HDD storage.
If you're willing to go bigger, there are going to be better deals in the HTPC realm, but it's hard to match this kind of performance in such a small package.