micro-atx is a smaller sized motherboard with only 4 rear expansion slots. It is a subset of a full atx dimension which has 7 slots, and should fit in any atx case, so you are good there.
Cases are a personal thing. May I ask what attracts you to the haf 912?
There are many attractive m-atx cases out there.
I use a silverstone tj-08E:http://www.newegg.com/Product/Product.aspx?Item=N82E16811163058
AMD wants a 540w psu with a 6 pin pci-e connector.http://www.amd.com/us/products/desktop/graphics/amd-radeon-hd-6000/hd-6770/Pages/amd-radeon-hd-6770-overview.aspx#3
Either psu you listed would do the job.
I would not worry about cable lengths.
On occasion, a bottom mounted psu may have to stretch over the motherboard to reach the 4 or 8 pin cpu power spot.
In the unusual event that one cable is short, there are extenders available for low cost.
You might also look at the corsair 500w unit:http://www.newegg.com/Product/Product.aspx?Item=N82E16817139027