Read this: http://www.htpcforums.com/index.php?showtopic=5876
It answers your questions.
Some personal observations, lower power AMD A6 APU is plenty powerful enough and quiet with the stock cooler. In testing my A6 box CPU is 20 degrees C in operation with low CPU and memory utilisation. You actually need less resource than expected. Integrated APU graphics designed for video which is an important consideration.
4G RAM is plenty and it doesn't need to be fancy or expensive. Do get an SSD for the boot drive as the device turns on and off almost instantly.
I bought the case with the integrated iMon and remote and it was a good call. I went win7 but customised heavily. It works just fine.
I looked at the Atom box solutions and Atom is less powerful and generally lacking in the graphics department. I think custom is better in many situations as you can get exactly what you need for a start.