I always go for the reviews to see what kind of power consumption I can expect:
http://www.tomshardware.com/reviews/radeon-r9-270-review-benchmarks,3669-9.htmlOne card on its own seems to draw a bit less than 150W (which makes sense given the one PCI-E connector). Therefore with 2 cards, you're looking at 300W. Add an overclocked CPU and other stuff gets you at still less than 500W. Therefore, in my opinion, no, you do not need a 1000W power supply. However, you still want a GOOD power supply, not the 250W PSU that claims to be a 600W PSU.
Indeed, a decent brand 600watt PSU should be enough. If you want to be certain, then take a 750watts.. Then you should definatively be safe, IF you use a good brand. Try to do some research on the internet because there's a lot of difference between the PSU's