Power!
A big part of building small form factor computers seems to be about figuring out how much power you will actually need, before trying to pick from a wide array of power options. I think there is a general trend where people over-estimate how much power thier system uses, insisting that anything under 500W isn't going to cut it. It turns out that is mostly wrong, especially if you choose efficient components and also strip away all of those bits that you probably don't need (optical drives, hard drives, additional PCI cards + case parts that require dedicated power).
After reading a bunch of bit-tech articles on CPU and GPU power consumption of various similar components, I calculated that anything above 300W should be sufficient, but a 400W or 500W would give my system more 'wiggle room' and operate more comfortably.
Pico PSU - I discovered Pico PSU early on. They have an external power brick that converts the AC mains to DC. Then an adapter inside the case splits the voltage to the required components. I think though, that the highest Wattage they support is about 160-200W, which simply wasn't going to be enough for me. Though, this looks like a reasonable choice for anyone with a HTPC-type system and doesn't have a dedicated GPU (or has very low power GPU). Seen here is an example of the power brick and adapter. Note the power brick isn't very pretty either!
SFX Form Factor - On the other end of the scale, Silverstone have some super powerful supplies in a small form factor. The smallest volume they have is 100mm x 125mm x 63.5mm(I think!), which is available up to 600W! For quite a while I contemplated using one of these supplies and was trying to figure out how I could orientate the supply and components inside the case, without wasting valuable space. Positioning the PSU next to the other components would waste space in at least two dimensions. Event positioning the PSU behind the other components would add extra height and depth to the console that I didn't want.
Flex ATX / 1U Power - Then I came across the standard form factor for server power supplies. These come in wide variety of shapes and sizes and you'll see the names "Flex ATX", "1U" or "2U" used to describe server power supplies. While I do know 1U and 2U refer to single and double height supplies, I'll admit I'm still unsure if flex ATX is the same as 1U!
Either way, they're all 40mm in height, which is very close to the 'flat' height of any GFX card you might see and also pretty close to the height of a motherboard with stock cooler and standard RAM. The width and length of these power supplies vary A LOT, but the smallest comes down to about 81mm in width and 150mm in depth - which again, is pretty close to the 170mm depth of a ITX motherboard and the 150-170mm length of a graphics card. They also come in Wattages from 100W up to about 500W (if you can get your hands on one). The 350W and 400W specced models are quite common though, and sufficient for what I wanted to do.
A 40 x 81 x 150 Flex ATX PSU
Be Aware that many 1U supplies are the larger 100mm Width or are very long!
Other 1U supplies also do NOT ship with standard cables or are hot-swappable units. Check before buying!
In the end I purchased a 400W Flex ATX supply from FSP. The power should hopefully give me enough room to operate comfortably without over-burdening the PSU. The manufacturer also came highly rated, so we'll see how it fares!
Last Note: I discovered HD-PLEX after I purchased the power supply. Which seems to be the basis other modders have built small form factor PC's with dedicated graphics. Another power brick style solution that provides much more power than Pico PSU. Something to consider for the future!
