Y helo thar!
A gaming SFF eh? You've come to the right place. It's certainly possible to build a beast of a machine into a small form factor. I'm currenly about to order myself a new system, including a 4870.
The biggest trouble I had was with the case. Being smaller than full ATX, they don't have as good airflow as their bigger brethrent, so you have to be more careful with fan and cooling choice.
The following cases are all great for a SFF gaming machine, i've listed what fans they take.
Silverstone Sugo SG01 Evolution - 3x80mm
Silverstone Sugo SG03 - 2x120mm
Lian-Li V350 - 2x120mm and 1x80mm
There are others too, but IMO these three are the best. The build quality is top notch, and they look the dog's danglies.
As for CPU cooling, here it starts to get tricky. You're not going to get anything like a TRUE in there, or even close. The Sugo cases have just 78mm clearance, and the V350 doesn't fare much better, allowing around 120mm. This means you're restricted to things like the Zalman CNPS8700 and Silverstone NT06 in the Sugos, although i'm probably going for a Scythe Mini-Ninja in mine (V350) - it should just about fit.
Beyond that, spec as you would a full ATX - just remember that the motherboard needs to be microATX. Full graphics cards should not be a problem, although I believe in the SG01 you'll lose the HDD bay if the card is longer than 9.5" - but you can just mount it in the spare optical bay.
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