This response is one of those "I was told" replies. Please confirm if the following is correct or amend as to what would work.
Also not wedded to 16GB of memory.
Was told - that the EVGA 015-P3-1580-TR GeForce GTX 580 1.5GB GDDR5 384bit PCIE 2.0 SLI Support Video Card does not fit onto a Micro ATX motherboard. Unfortunately I do not know if that statement is correct.
If the EVGA 015-P3-1580-TR GeForce GTX 580 does fit on a Micro ATX motherboard, what are your thoughts on a mother board with reasonable power and features similar to the Gigabyte GA-Z68XP-UD4, with 16GB memory (4 banks)?
Regardless, you can expect any pci-e graphics card to fit any motherboard with a X16 pci-e slot. It is a standard.
What you might find, is that many M-ATX cases can not hold a long graphics card. The GTX580 is, I believe 10.5" long
As to how much ram, the norm is a 8gb kit of 2 x 4gb of DDR3-1333 or 1600.
If you run 64 bit enabled apps that can make use of lots of ram, like photoshop, then 16gb kit is ok.
Why not? Ram seems so cheap today. Just don't pay much extra for faster speeds or fancy heat spreaders that are not needed.