Well if thats the case then you have nothing to worry about in terms of straight up power usage. Even if you had the most power hog CPU and system right now you would still be perfectly fine running any of the GPU's mentioned. Power usage is sometimes blown out of proportion when talked about and sometimes the constant mentions of one or the other using more power is blown up along with power requirements for a system. With a 600watt psu even though it's only 80+ rated (after reading its base specs to make sure) you could run a pretty heavy system with it without fuss. Let's say you are running a gaming rig that is a pretty heavy hitter (i7 and what not). An i7 3770k (just picked this CPU random) uses 140 watts roughly under load. Then include the motherboard, ram and hdd which on average would bring the use to around 200 watts depending on components and what not. Using that as just a basic rough estimate let's say we play safe and assume an extra 50watts headroom (for just in case scenarios and overclocks) that is still just 250watts. That would mean mixing your psu in that machine would leave 350 watts laying around if we said your PSU could run at 600watts constant. Of course you would not want to do that but you should get the idea. My numbers are very rough btw, I'm just basing it off of know and average power consumptions I have seen measured. So over all what I'm getting at is that you have plenty of room for an even higher GPU even. Heck I saw a rig built recently using a 500watt psu and an R9 290. But just in case you want to see for yourself here is an example system load comparing total power usage of the same system only swapping video cards You can see that truthfully computers do not need as much power as sometimes people are lead to believe.