Out of Pod Experience

Really? I thought you could use the 3rd and 4th DVI to run the last monitor.

http://www.xbitlabs.com/articles/graphics/display/amd-eyefinity-nvidia-surround_3.html#sect0

3GB 7970 eye finity should be your best bet if the main concern is TDP. Basically it will run lower under normal usage at 210watt.

However, at max load, which it seems like you'll be permanently at you're looking at between 400 and 550w. (http://www.hardocp.com/article/2012/02/08/gigabyte_radeon_hd_7970_oc_video_card_review/8)

It's very very important that you get additional cooling and if you consider a water block or something then this will probably be the best card. This is because temperature increases resistance which means you need a LOT more power to drive a component.

With a water cooled 7970 you'll be looking at about 300-350watt power draw on a non OC 3GB 7970. 1GB per screen should be plenty.

However if thats not an option, definitely go 680GTX for power draw, it maxes out at less than 400w on air. http://www.anandtech.com/show/5699/nvidia-geforce-gtx-680-review/19

Finally, if you cant handle the thought of 768mb per screen (remembering that if you are streaming video on one then most of the GPU RAM will go to the other two) then the 690 is an option, however, it is much much more power inefficient. with a TDP near 680's Max power level I don't recommend it. It still will be more efficient than your current setup, by about 20%, but to me its just not worth it as it still may be slower over 3 screens. (Due to the bandwidth bottleneck of using only one PCI-E and the software side splitting of screens. Geforces are very good at doing it for 2 screens but it remains to be seen how they handle making their cores triple threaded, I know ATi is better at that)

as regards the 680 and all the kepler cards, they support 4 monitors so it's no problem to sli them in a 2-3-4-way and let 3 monitors work at the same time. As regards the 580, and any of the 5xxx cards, that configuration is possible using the 3-way sli, since its peculiarity is to let the system recognize 3 cards instead of 1 as it is for the 2-way sli. That's also why you must plug each monitor to a single card and you cannot plug 3 connection on 1 and 0 to the others. It would be interetsing to know if you could use 6 monitors with a 3-way sli of 5xxx cards, since they support 2 monitors each so theoretically you should be able to plug 6 monitors. And also to compare that configuration with a 3 cards one but not in sli ;)
But we're dealing with 2 cards here so there's not such an option.
Moreover there's another limitation to the 2-way sli/xfire (tbh I dunno if there's such a limitation in the 3-way sli but prolly not): if you pair 2 cards with 1 Gb ram each, your system will still use 1 Gb ram after the sli. That's mainly why the performance increase are far from being doubled. From what I read in the benchmarks around it doesn't go any further than a +20-30% at max and why imho it's just a waste of money.