Can someone explain why when you upgrade your imac from the apple store, there is such a big difference in buying 8 gigs of ram? For instance: 8GB 1066MHz DDR3 SDRAM - 4x2GB Add $188.00 versus: 8GB 1066MHz DDR3 SDRAM - 2x4GB Add $564.00 Its still 8 gb!!!! Why the big difference? Do two 4gb work so much better then four 2gb? I'm sooo confused!