First, if you haven't done it already, thoroughly read the water cooling guides before replying in this thread, it will save you a lot of time and questions!
Two 7970's would need at least 360mm worth of decent rads to keep them nice and cool (at least 3.5cm thick). Which, depend's on what case you have and if you can fit them. EK and XSPC are great makes which OcUK have a good range of radiators in different sizes.
Fans are just as important, Gentle typhoons and Corsair SP120's are great radiator fans!
Reservoir doesn't really matter, the only thing you need to do is make sure it goes before the pump and preferable higher up than it.
Most blocks today are high flow and most pumps are quite powerful, so choosing a pump these days to suit your needs and that offers room for loop expansion has never been easier. Two high flow water blocks will be fine with anything over 400lph, most pumps are at least 700LPH. Having a super high flow pump does not give you better temperatures if your water is already flowing fast enough to keep the loop temperature uniform. In a two block set up, you will find the performance difference between one 800lph pump and two 800lph pump's negligible (it will just be noisier). Most pumps need a pump top which can make the cost of your loop add up. You can save on the pump top buy getting reservoir pump combo's like Bay reservoir/pump combo's sold by XSPC. Alternatively you can get EK Water-block's 4.0 DCP pump which needs no pump top, is cheap and pumps at 700LPH.
The size of the tubing you choose is personal preference. Anything over 1/4" internal diameter has little effects on temperatures. Clear tubing will stain over time if you use coloured coolant, you can avoid this by getting coloured tubing.
Which fittings you choose is up to you, as long as they fit your tubing! I personally avoid Monsoon fittings, though they are beauties, i find them a bit fiddly and prefer compression fittings with thumbscrews for ease. I recommend using compression fittings for your first time as they are a little easier to install, but using 1/2" barbs on 7/16" ID tubing with Koolance clips can be just as secure (just a little more difficult to install).
As for blocks:
http://www.coolingconfigurator.com/
Go here to get a list of compatible EK blocks. Getting a universal block will let you use that block when you change GPU, however your RAM would be left uncooled and you might find that it gets too hot without a full block or some air flow.
Thermal geese!!! Im mentioning TIM because, though obvious, its super important that you get non electrically conductive TIM for GPU's.
I personally find it odd you are willing to go xfire with a custom loop but wont include your CPU in the loop. I know you have a H70 but a custom loop can perform better and if your spending so much money and effort watercooling your GPU's, why not sell your H70 and get a bit more radiator and a CPU block.
Also:
+1 to both PGI947 and Notorious
