Building a new rig and the Sapphire r9 290 sounds like a good bang for buck GPU. But with the rumors of the GTX 800s coming soon im debating whether or not to go amd if the Nvidias come out before November.
unless you need the upgrade now i would wait. the r9 290 isnt going anywhere and will probably not increase in price (if not decrease) and if the 800 series has a card that is better then you will not be regretting your decision.