Oh, sorry i missed the dual...
Give me a sec...
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OK, here is how it goes...
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More or less a 2 x R9 270X is faster than a single R9 280X.
But it depends on the game actually. If a game supports a multi GPU, an a crossfire configuration you will get more fps compared to a single R9 280X.
For example on Metro Last Night and Hitman Absolution the differences are not very high, even those two games supporting multi GPU
At 1920x1080 the difference will be between 5-17 fps, depending what GPU enhancements you will choose to use. (AA - AF and so on)
On the other side if you choose to play Tomb Raider or Sleeping Dogs, then the benefit will be much higher. !!!
(25-65 fps more on Tomb Raider, and 10-30 fps more on Sleeping Dogs)
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Keep in mind you should have a PSU with a good stamina for 2 x R9 270X GPU's.
Minimum 700 watt. !!!
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How much have you found the system with the Dual R9 270X. Near 1000 bucks more or less ???
You could build it for near $ 1100. (http://pcpartpicker.com/p/KRwfnQ)