The Gigabyte board chefjw recommended will allow 3 way SLI or Crossfire. The DFI board you listed will do 2 way. It would take a chunk of change and a substantial power supply to do 3 way though, and tests show it may actually reduce performance.
I generally prefer the ATX boards over the Micro ATX. Just more room on them.
As far as the video card goes. You can probably pick up two GTX 260 cards for close to the same price as a single GTX 280. SLI 260's will beat the 280 all day long (at least in games which support SLI).
I haven't looked at to many 275 benchmark/reviews personally. My GTX 260 is a Core 216 model, and runs everything I throw at it.
Other thing to look at, is the resolution of your monitor. I play stuff @ 1920 x 1080 and my GTX 260 is plenty for me. My GTX 260 is an MSI made unit. At it's stock fan settings the thing is near silent. Though, it admittedly needs to have the fan set higher to keep it as cool as I'd like it to be.