I have a 4th tier PSU (Corsair CX600). Should I get the GTX 970, or the R9 390?

[Yaqzan]

The R9 is the better card, but it uses ~100W more than the 970. Using the 390 with my PSU should be fine, but overclocking is out of the question. However, with a 970, it's low power consumption means I can overclock it to the point where it uses as much power as the 390, and probably get better performance. What do you think?

 

[Luminary]

For the CX600 I wouldn't pair anything over the GTX 970.

You're quite correct with your assumption about the R9 390's greater power draw. The CX line really isn't designed to perform well with the heavy draw of a card like the R9 390, and I wouldn't advise running that card unless you also were upgrading the PSU.

You should be fine with a GTX 970, though obviously the BEST solution would be to upgrade your PSU as well while getting one of these higher performing cards.