Hmmm, kind of hard for me to determine exactly what wattage would be needed. I've never ran dual GPU's before, so I'm not really sure on the ins and outs of doing it. That being said, I notice that when reading some reviews on Newegg, several users stated they were running R9 290's and 290X's on a 1000 Watt PSU. I went over to the extrem power supply calculator tool and plugged your specs into it adding the 2 280X's. It calculated that the system would use 768 Watts @ 100% load. Going by this, a high quality 850 Watt PSU should be up to the task. Now if you're a heavy overclocker, or plan on running that route in the future, I'd say a 1000 Watt PSU would be the best bet to ensure you dont overload the PSU. Looking at the Corsair RM series on Newegg, the price difference is only $10 from the 850 Watt to the 1000 Watt. Personally, I would go for the 1000 Watt simply to ensure I would have no problems in the future. So in short, a quality 1000 Watt PSU for the extra overclocking headroom, or a quality 850 Watt for plug and play operation. Just be sure to get a certified one. Most PSU's in that power range are gold Certified at the least if they are certified. The ones that aren't should be avoided. Corsair, XFX, Seasonic are a few good brands which I've had great luck with over the years.
So far I have been very happy with my 280X. It has been overclocked to 1170 MHz on it's core with a slight voltage bump, but even when it was stock @ 1070 MHz, it was still very fast. I do play on a 23" 1080P monitor, but I don't play those 2 games unfortunately. I can tell you it handles Metro Last Light, Tomb Raider, and Wolfenstein New Order all on max settings. Assassin's Creed 4 required some tweaking in the control panel to overcome Ubisoft's badly optimized V-Sync, but after tweaking it runs that game at maxed settings except for AA and God rays, which is at 4X and Normal respectively. So far I have yet to come across a game which cannot run on the highest preset on this card. Some will run at less than 60 FPS when more visual candy is enabled, such as Tomb raider when 4x SSAA is enabled (I hate FXAA) runs at around 40 FPS or so. I'm pretty happy with those results. Originally wanted an R9 290, but had to compromise to a lower price point. At the price point the 280X is at, there isn't much else out there. The GTX 770 lost out for me simply because it has less VRAM, and I was not about to fall victim to the whole "2GB is more than enough" campaign. I fell for that once before when "1GB is more than enough" was going around, and we all know how that went.