OP, I have had dual air-cooled 590s so I feel I can give you some solid advice on this question of yours.
I bought the EVGA GTX-590 Classified Quad-SLI pack when it came out in March. I installed them in my system with one extra PCI-E slot between them in order to give them some breathing room.
My "regular" fan profile was set to 75% and for intensive applications, work, and gaming, my fan profile was at 95%.
The hottest any of the GPUs got was 85C during gaming, and that was at 8100x1600px (3 30" monitors in Surround mode).
The cards were rock solid and the EVGA Classified looked beautiful with the jet-black theme.
The only reason I sold them and got 3 EVGA GTX-580 3GB was for the VRAM (3GB per GPU) so that I can bump up the AA while gaming at insane resolutions (8100x1600). I was able to game without any hiccups at that same ultra-high resolution with the GTX-590s but I had to turn down the AA to about 2X and in some games (like Bulletstorm) to 0X. As far as processing power, these were some of the best cards IMO. I've done a lot of photo and video editing and these GPUs really helped a lot.
I used to have a GTX-295 CO-OP until January of this year so I know where you're coming from.
The answer to your question really lies in the answers to the following questions that pertain to YOU:
1.) What monitor(s) are you going to be using?
2.) Are you going to be running NVidia Surround or AMD Eyefinity?
3.) What kind of applications will you be running mostly?
4.) Is this rig strictly for gaming?
5.) How important is overclocking the GPUs to you?
The answer to #5 is especially important as the GTX-590s, especially the EVGA ones which come factory OC'd to 630MHz from the stock 607MHz per GPU, do not overclock that well since the power circuitry (# of phases for the GPU/Memory) has been uniformly lambasted by reviewers and owners alike. In order to OC a card, the power circuitry, or VRM, needs to be robust. The GTX-590 has a 6-phase (?) VRM-design which is not ideal for OC'ing the card.
The greatest advantage of getting Quad-SLI GTX-590s is that it takes only 4 slots on the motherboard (2 per card), well, 5 if you count the slot inbetween, and you get 1024 CUDA cores and 4 FULL GF110 (580) GPUs. If you do number crunching, folding, or some sort of analytical work or research, the GTX-590 Quad-SLI is a great setup.
If you want the best GAMING performance and even future-proof it to a certain extent with regards to multi-monitor setups etc., go for the 3GB GTX-580s.
I'm assuming since you're considering the dual GTX-590s, money is not that big of an issue so decide after answering the questions above carefully.
Asus is supposed to come out with a beefed up GTX-590 with extra VRMs and OC'd from the factory. Wait for those to come out and make up your mind!