GTX 780 or R9 290?

Neither. Get the GTX 970, which is better than both and only costs $350. Or, the GTX 980 which is the fastest single GPU card around for ~$550-600.

PCPartPicker part list / Price breakdown by merchant

Video Card: Asus GeForce GTX 970 4GB STRIX Video Card ($349.99 @ Amazon)
Total: $349.99
Prices include shipping, taxes, and discounts when available
Generated by PCPartPicker 2014-09-22 19:47 EDT-0400

or

PCPartPicker part list / Price breakdown by merchant

Video Card: EVGA GeForce GTX 980 4GB Video Card ($549.99 @ Amazon)
Total: $549.99
Prices include shipping, taxes, and discounts when available
Generated by PCPartPicker 2014-09-22 19:48 EDT-0400