Get a Gtx 780 it is same price as a R9 290x and the performance it a bit better on a single screen.
But for $100 less you have a card that is about the same as the R9 290x which is a great card too:
PCPartPicker part list: http://pcpartpicker.com/p/QvmZJx
Price breakdown by merchant: http://pcpartpicker.com/p/QvmZJx/by_merchant/
Video Card: Gigabyte Radeon R9 290 4GB WINDFORCE Video Card ($379.99 @ Amazon)
(Prices include shipping, taxes, and discounts when available.)
(Generated by PCPartPicker 2014-06-05 12:46 EDT-0400)
You can overclock the R9 290 to the performance of a stock R9 290x so if you are on a budget this might be the best option.
But out of those two I would go for a Gtx 780 since it is cheaper then a R9 290x and it is a bit better.