swap out the motherboard for this one.
GA P55-US3L Gigabyte P55-US3L 1156 ATX
i don't know why that ram from amazon costs so much, but get this set instead. it's cheaper and it runs at the same speed and timings.
CMX4GX3M2A1600C9 Corsair DDR3 1600MHz 4GB kit of 2x240 pin 0DIMMs, Unbuffered, 9-9-9-24, XMS3 with Classic Heat Spreader - Core i7, Core i5 and Core 2
also, atleast from that site you're using, you can actually get an antec 750w modular for slightly less money than the corsair 650w, so you might as well.
regarding some of your questions:
no you won't need to buy a heatsink for the cpu, it will come with a stock one. if you're planning on OCing though, you should buy a better aftermarket heatsink.
no, you won't need any additional cables to get your build set up.
an atx case will fit both atx and m-atx boards, and yes the psu would work with an m-atx board, they use the same power connections as a standard atx board, it's just the size and layout that's different.