Unfortunately I'm pretty sure that a u-ATX won't fit inside an A1200 case, you might be able to squeeze one inside a 500, I don't know, never checked the measurements, but I reckon M-ITX is the largest form factor that could comfortably fit inside the 1200 case, plus with it's diminutive size it would allow ample room for a gfx card.
EMT64 is Intel's attempt to rebrand AMD64 and pretend they came up with it, AMD64 was AMD's answer to Intel's Itanium, and was a much smarter idea than Itanium (Itanium was a whole new architecture, AMD64 was just expanding the x86 to 64 bit) so if you're going to use an Athlon of any kind, it's not an EMT64 chip, but an AMD64. Or if you're a Microserf then both are grouped together as x64, which sounds much simpler really.
A riser is a device that attaches to an expansion port of whatever kind and simply extends the port to a new position, for example there are clockport risers for the 1200 that move the port connector up for use when other devices might block the port in its original position. PCIE risers have flexible cables that enable you to position a PCIE card how you wish, rather than being tied to the right-angle arrangement of normal PCs, useful in x86-in-Amiga conversions as obviously right-angled PCI cards wouldn't really fit inside a wedge Amiga case.
