I had the same question when the 290x launched, so here's my opinion since i went a slightly different route.
if you get an 290x with an aftermarket cooler you should have no problem at all with heat.
performance wise just one card a 290x will always be faster then a 280x.
that being said, just one 280x will still be enough to run BF4 & watch dogs on 1080p max detail without issues.
in the end, i bought two 280x cards for less money then a 290x.
(i still can get two 280x Cards for less then a 290x even at this point)
its not a setup I recommend to anyone, but the performance better then a 290x, price is less.
you do need a bigger power supply, a nice big case with decent airflow,
edit: not sure why i replied to this thread without checking the date