I think you were told wrong.
Here is the GTX580 I have:
EVGA SuperClocked 015-P3-1582-AR GeForce GTX 580 http://www.newegg.com/Product/Product.aspx?Item=N82E16814130590
For what it is worth, the AR suffix is lifetime warranty, the TR is 2 years.
The motherboard I used is the ASUS P8P67-M PRO (REV 3.0) LGA 1155 Intel P67 http://www.newegg.com/Product/Product.aspx?Item=N82E16813131707
I was an early adopter, ant it was the only P67 M-ATX motherboard out at the time. It supports sli, but cooling will be an issue with the two cards necessarily next to each other.
Moot point since I have no need for sli.
The motherboard has been superceeded by a couple of new ones. Z68 based.
The ASUS P8Z68-M Pro looks good @$129:http://www.newegg.com/Product/Product.aspx?Item=N82E16813131786
The ASUS Maximus IV Gene-Z is the current equivalent @$169:http://www.newegg.com/Product/Product.aspx?Item=N82E16813131759
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.