sometimes when using crossfire there is very little performance boost, i was using 2 r9 270s but decided to sell them both and just use one r9 270x now, there isnt much difference
if you are going to be using it for gaming, buy the r9 270x, if its just for doing work or general use get the r7 260x. im currently using a 270x and am very satisfied with its performance
there isnt much difference between the 2 cards, the 270x has a slightly higher clock speed and slightly increased performance. i personally use a 270x and can say it meets my expectations.
yes the motherboard takes several different speeds of ram, the ram you have chosen is compatible with most motherboards and a quick check told me it is definately compatible