7970 6gb watercooling?

One of the only reasons to pay a premium for one of these awesome cards is because they already come with amazing cooling. To my knowledge, there is no water cooling that would work on either of these cards.

You would be better off getting a reference 7970 if you insist on water cooling.

Oops shit, soz I had tons of pages open :S my bad XDDD ^^ good think you noticed.. (but ye, imo the best option for you would a GTX 690 4gb rather than the toxic. But I will help with the toxic)

The video tests show that the VRAM usage never peaked over 3gb, for 2500x1600, so would have been fine with a 4gb for 5760x1080 (its a roughly a third in the increase of pixels having to be rendered, so by estimation a 4 gb card would have been enough).
Regardless my main point is one 7970 toxic on its own will never be able to play crysis 3, maxed on full settings over 3 screens, where as 2 of them may just do the trick... so if your really that keen on 6gb >.> get 2 7970 toxics (phew that's costly XD)

So cooling the toxic eh? well there are a few ways of going about it..
Now I'd say the best thing is to buy the card first and get it home, open her up and have good inspection of it (using online/even past experiences is not good enough since PCB changes are made very often).
Then post few images of it for us, and we can help you out in more detail.
But in general, you will need few things:
-a universal block
-heat sinks of many sizes (order these in excess. since you never can be sure)
-obvs thermal pads for the heat sinks and tim for the block
-good air flow over it, for most purposes this means finding a way a stick a fan on top of it

Now we the universal block it is quite simple, you just find a block which compatible with a 7970 gpu (the GPU size will still be the same);
e.g. if you like EK aesthetics - which I personally dislike but most people die for XD - you could try these out:
http://www.ekwb.com/shop/blocks/vga-blocks/universal-vga-blocks/ek-vga-supremacy.html
But again this has all been done by a "visual" compatibility check by EK, so its really best if you order the toxic, measure the GPU size and double check that the block will cover all if it, but the GPU really shouldn't have changed, its more the PCB and VRAM that might have been jiggled about.

Here comes the hard bit... cooling the inductors, vram, mosfets, e.t.c
e.g. the size of this thingys vary (as manofchalk linked you the in the sticky, there is a basic explanation of what to do)
But thats why I'm saying you should order in a few heatsinks, rather than the minimal amount.
Or at least have gotten the card first, identified and measure all the thingys you need to cool, then order the heatsinks.
Here's a few I ordered for mine:
http://www.specialtech.co.uk/spshop/customer/Enzotech-Passive-Mosfet-Cooler-10-pack--MOS-C10-pid-13321.html
http://www.specialtech.co.uk/spshop/customer/Akasa-Low-Profile-VGA-RAM-Heatsink-10-Pack-Black--AK-VMC01-BK-pid-14060.html
It shouldn't matter too much if you use VRAM heatsinks for cooling the mosfets or vice versa, all that matters that you get some heatsinks on the thing your try to cool and it to cool all of it.

With the thermal pads, most heatsinks will come with self adhesive thermal pads, which would be good to use, but it they don't you could try these (1mm is fine for most applications, 0.5 mm for the capacitors but they usually don't get that hot anyways):
http://www.specialtech.co.uk/spshop/customer/Phobya-XT-100-x-100-x-1mm-Self-Adhesive-Thermal-Pad-pid-14138.html

After your done with all the fitting of the heatsinks and gpu block, then we you try and boot, you could get a infrared heat sensor (if your paranoid), and just stress test the GPU to see if any temps are unreasonable.
If you don't have that you can use the good old finger touch method: touch a few of the heatsinks: they warm -> tis good, they cold -> mybs a problem)

If that's all working nice, then you will have successfully cooled you block with a universal gpu block.