You could get a R9 290 too and overclock it. It is 160$ cheaper and just as fast as a stock R9 290x when overclocked.
PCPartPicker part list:
http://pcpartpicker.com/p/WzJMFT
Price breakdown by merchant: http://pcpartpicker.com/p/WzJMFT/by_merchant/
Video Card: Gigabyte Radeon R9 290 4GB WINDFORCE Video Card ($319.99 @ Newegg)
Total: $319.99
Prices include shipping, taxes, and discounts when available
If you really want the R9 290x then get this one it is cheaper then the one you listed and just as good, though the extra performance the R9 290x offers over the R9 290 doesn't justify the 160$ price difference:
PCPartPicker part list:
http://pcpartpicker.com/p/g66VWZ
Price breakdown by merchant: http://pcpartpicker.com/p/g66VWZ/by_merchant/
Video Card: Gigabyte Radeon R9 290X 4GB WINDFORCE Video Card ($485.99 @ Amazon)
Total: $485.99
Prices include shipping, taxes, and discounts when available
I would recommend you to go with an R9 290 and overclock it.
Don't even bother looking at the Gtx 780 and the R9 290x they are both overpriced compared to the R9 290.