A. It depends on what resolution you are playing at and what videocard you have. Usually you can expect a boost, but if you play at 1600x900 you will not see a difference between a single gtx 970 and a gtx 970 sli setup, since 1 gtx 970 is already overkill for that resolution. Also if you have a weak CPU it might bottleneck the sli/crossfire setup.
B. In sli you can use the other cards output ports, in crossfire you can only use the primary card's output ports. It will not influence performance.