Well, the next step of the road is here, we've built a 12-gallon sized computer that uses an ATX board (P8Z77-V Deluxe), an overclocked Intel Core i5 3570K with dual eVGA GTX-670 in SLI. It's been running fine (except I can't get the pump to throttle). Here are notes on the first step of building a 5 gallon computer from a mini-ITX chassis.
The bill of materials mainly gotten through Newegg (btw, use the united mall, to get double miles when you click through)

• Silverstone SG06-450. 450 watt SFX power supply
• ASUS P8Z77-I Mainboard
• Intel Core i5 3570K Processor
• Maxstor 120GB SSD
• Samsung 2x4GB DDR3-1600 Memory
• Noctua NH-L12 Cooler

Here are the installation notes and order, it isn't super obvious because we are tryingt o use a big cooler on a tiny board

• Open up the Silverstone, remove the cover with the four screws in the back and slide it back
• Remove the 3.5" carrier and the optical drive
• Put the Noctua backing plate behind the motherboard. Remove the rubber insert in the middle and make sure it lines up with the stock backing plate. You don't remove the stock one.
• The hardest part of the direction of the cooler as it is nearly as big as the mini-ITX board. Try as we might, we could not use the 120mm fan in the lower position, it kept hitting the ram, the voltage modules or the capacitors. Ugh. Only way was with the 92mm fan on (not as good for cooling). Seems like the two solution are either to shove the 120mm fan to the side so it fits or get a slim 120mm fan so it can clear the memory (probably that's the best answer).
• The rest of the install is pretty straightforward. Install the cooler with the heat pipes towards the PCIe slot

So the big problems to solve are:

• Enough cooling either by moving the 120mm fan or by a thin 120mm fan. However, running Prime95 at full bore, the CPU temps get to just 60C running non-overclocked. That's not quite as cool as the big micro-ATX sized P8Z77-Deluxe with Noctua NH-14 running at 50C at full bore, but pretty amazing for using just the 92mm fan.
• Overclocking is a little confusing. We got to 4.4GHz with 43 multiplier runing at 103% of base clock (103MHz) with voltage increased to 1.3V it looksl like. And the H4000 onboard graphics is overclocked 13% to 1.3GHz. But the DDR3-1600 ram isn't overclocked, some say that it can run better with hand overclocking and I need to find a good manual guide.
• Dell Ultrasharp U2412M. It is just $300 and is a very decent monitor with all the fixings. It is an IPS panel so much brighter than the cheaper TN panels. It is fine for gaming. The ASUS PA248Q is a close second and support 1:1 mapping which is great for XBox and other applications. See Wirecutter.com and FlatPanelsHD.com • Dell Ultrasharp U2312HM. It is 23" so not visibily smaller than the 2412M, but it is just$250 and is also an IPS monitor