why is 970mobo 50$ cheaper than 990

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Because the 990fx motherboards use a more advanced chipset with better features and better parts in general than the 970 boards. They overclock better as well. Usually, the 970 boards are under 80 dollars, and the 990fx around 120, so I wouldn't bother paying 220 for a 990fx. At that point, you'd be better off going with a 130 dollar intel motherboard and Cpu.
Because the 990fx motherboards use a more advanced chipset with better features and better parts in general than the 970 boards. They overclock better as well. Usually, the 970 boards are under 80 dollars, and the 990fx around 120, so I wouldn't bother paying 220 for a 990fx. At that point, you'd be better off going with a 130 dollar intel motherboard and Cpu.
 
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