1. There is no such thing as a Micro ATX Graphics Card. However, Micro ATX motherboards WILL fit in a ATX case, but ATX motherboards WON'T fit in a Micro ATX case.
2. As long as the motherboard has PCIe X16 slots, and your case can fit the graphics card, yes.
3. The difference between ATX and Micro ATX is that Micro ATX is smaller; It is slightly shorter than a ATX board. Micro ATX motherboards have fewer PCI/PCIe slots and are designed for smaller cases and small-form-factor cases, but will work in any normal ATX case.