well its recommended to have at least a 600W psu for only ONE GTX 580, so I'm assuming with with a TDP of 244, you will be getting awful close to pushing your PSU to the max, I think you could get away with it though.
...that's assuming you will actually ever be stressing all your components at once. If you have GTX 580 SLI there won't be any games that will come close to stressing your components.
I'd recommend waiting for a few weeks and seeing what AMD has to offer with the 6900 series, then you can get two of those without having to worry about stressing your PSU. Or just get 1 GTX 580 for now and when you realize that its completely overkill then you might not want the second anymore.
But if you do plan on sticking with the GTX 580's, then you may want to consider getting a higher watt PSU to be safe, just remember, do NOT skimp on quality.
Well I can understand why you might want to upgrade because I've heard about the shoddy drivers with the 5970.
This is just me being frugal but I'd suggest getting just 1 GTX 580 to start off with, and see how you like it, if its not giving you the boost you were looking for, then go ahead and upgrade your PSU and buy another 580.
Again there is always the option of holding tight for a few weeks and seeing what the 6900 series has to offer. If anything it will drive the prices of the 580 down a bit . Also I know if you crossfired HD 6970's you probably wouldnt have to upgrade your PSU.
Just putting it out there for consideration
EDIT: Obviously you are looking for the best gaming experience possible by getting the fastest graphics options available. If you would like to make your gaming experience even better, I would recommend adding another 2-4GB of RAM. You won't notice much of an FPS increase, but YOU WILL NOTICE IN GAME, it will lead to improved loading speeds, higher detailed levels and better-quality textures, as well as less artifacts. Again, just a suggestion, best wishes to you.
Guru3d recommends a 900+ watt power supply for GTX580's in SLI, more if you overclock. Having said that, they run their tests using a highly stressed system, so you would need to determine how stressed your power demands are. A lot of overclocking will push you over the limit with an 850 watt PSU.
The great thing is, you have one of the best PSU's on the market. It has been tested to consistently pull much more power than it's rated maximum. When I purchased my Corsair 850HX, at least a couple reviews said it was the best power supply they had ever tested. I would say that your PSU can handle two GTX580's in SLI. Overclocking will not be necessary, unless you are trying to set a personal record. If you run into problems with power, then you could think about upgrading your PSU.
In terms of waiting for the AMD 6900's, I am also just a few days away from ordering a GTX580 for $499. I don't think the prices will be coming down on the 580's, nor would I consider taking a step down to the 570. I am not in the market for an AMD card for various reasons, one being I actually play several games that feature PhysX. So, for me, there is no reason to wait... unless Nvidia announces the pending release of the dual-GPU card that has been shown in several leaked photos: